Twins Paradox - Does ageing depend on motion? I am trying to understand the Twins Paradox (relativity) and its implications.
Can we infer from the Twins Paradox that the ageing process (cell decay or other biological processes) depends on motion? When I move fast, will my rate of ageing slow down?
I know the theory says that its time which slows down but my understanding (it may be wrong) is that time is just a human perception, it isn't a real physical entity. If time isn't real, outside human perception, then it must be the biological process of ageing being impacted by motion, in the Twins Paradox experiment.
Please help me, what am I getting wrong?
 A: Actually when you move close to the speed of light $c$ you don't notice any change in your internal clocks (heart beating, aging, etc). But those observers who see you travel close to $c$ measure that your internal clocks are slow.
A: You're touching on multiple issues here, and none of them has as simple an answer as you probably want.
Any notion of the statement "time is just a human perception, it isn't a real physical entity" is really philosophical rather than physical - ask a philosopher if you care to understand the issues behind such a statement, or even the deeper meanings of such words. Physics can provide certain, more concrete, but potentially less meaningful, answers. For what it's worth, physics can tell you that time is as "real" and "physical" as is distance, or anything else that you care to measure.
Anuar is right in that observers who see you travel close to $c$ measure your internal clocks as slow, but let's give that some clarification. First of all, since you're touching on fundamental, philosophical issues of relativity, let me state explicitly that the effect Anuar mentions is in no way an effect of human perception. It is a fundamental, physical reality of the universe. Relativity tells us that processes which are not perceived by humans and which in no way are influenced by human perception will proceed as though the aging of a person (or anything else) with a nonzero relative velocity $v$ is slowed by a factor of $\frac{1}{1-v^2/c^2}$. If any external object (conscious or not) - let's call that object Bob - is traveling in a spaceship at a signifcant fraction of $c$ relative to you, then compared to the clock on your wrist, the clock on Bob's wrist will be slow. 
It may be helpful to provide some relevant context to the notion of "clock" in this case - since your question seems to center around biology, let's define it in terms of biology. Fundamental biological processes - the Krebs cycle, let's say - proceed under given conditions (NTP, for example) at certain, fixed rates. Let's therefore define one Krebs cycle as a tick of your clock. It is important to note here that there is no universal, "right" notion of time. Time is always measured relatively to something. The universe does not allow you to define any notion of time that is independent of any physically realizable clock. Therefore, the Krebs cycle is as good and as real a clock as we could ask for, or as we need. Relativity says that if you witness another person moving past you at a significant fraction of $c$, the ratio of the rate at which that person ages to the rate at which that person's Krebs cycles proceed will be the same as that same ratio for you or for anyone else, but that you will measure all processes, including the fundamental, physical/chemical process of the Krebs cycle, and every other conceivable standard by which you might measure time, as slower than they are in your own body. There is not, even in principle, any means by which you might discern which person's "rate of time" is "right." Whether this effect is "real" or not is not a physical question.
Coming back around to your original question, the answer, unfortunately, is that it's largely a matter of the definition of the words that you're using. The rate of aging for each twin in the paradox, measured relative to any meaningful notion of time measured relative to himself, such as the rate at which the chemical processes in his cells proceed, is the same. On the other hand, each twin will measure the rate of aging of his twin to be slower than that of himself. Both of these statements are completely, objectively true. It is worth noting, however, that the classic twins paradox is formulated in the context of special relativity, which cannot account for the actual process of at least one twin turning around to go back to his brother, and that special relativity cannot answer the question of which twin will be older when they are reunited, and that general relativity is both necessary and sufficient to resolve the paradox.
A: Your cells always age at exactly the rate of one second per second in your comoving inertial frame; they ultimately are governed by the same physical processes that define regular intervals in a clock constructed as a timepiece. If the clock stays at rest relative to the old man in the song, "My Grandfather's Clock" will always tick the same number of times to number "ninety years without slumbering" to span exactly the lifetime of the old man, no matter what the path through spacetime of the pair may take. The point is that time is defined by the rate of physical processes and in our universe we observe that the ratios of the rates of these processes is the always same whenever they take place in the same inertial frame, be they pendulum swings, chemical reactions, intervals between the cell division and apoptosis events and the ageing processes that these imply or even the neural processes that define human perception itself; perception itself is a physical process, and it is not meaningful in physics to say something is "just a perception". So you always feel like you age at the same rate relative to what happens in your immediate neighborhood in the World.
Just as the ratio of rates of any pair of physical processes is always the same when the processes are in the same inertial frame, the same ratio will vary when the two processes happen in different, relatively moving inertial frames (or, more generally, when their instantaneously comoving inertial frames differ); the variation is governed by the Lorentz transformation and its implied time dilation factor. This is the fundamental meaning of time in physics, and we observe that it doesn't matter whether the rates are those of pendulum swings, chemical reactions or whatever. It's the experimental fact that these ratios are independent of the physical processes that allows us to have a well defined notion of time in our World.
A: When you are moving fast, your aging will slow down - see the Minkowski diagram: There are 3 worldlines of spaceships traveling in t=10 minutes (Earth time) 1, 2 and 3 lightminutes. The proper time τ is decreasing from 10 minutes to 9,54 minutes. The ratio τ/t decreases with increasing velocity. That means that when you are moving fast, your aging will slow down.

But wait a minute, there is just a small issue to resolve: velocity is relative! That means, if the spaceship claims moving (according to Earth frame), Earth could also claim to be moving (according to the frame of the spaceship). Who is aging less, Earth or the spaceship?
Physics offers two solutions for the arbitration of this conflict:


*

*The first solution is to refer to a preferred reference frame (in particular: comoving coordinates). The result will be that, according to the preferred reference frame, not Earth but the spaceship is moving near light speed. 

*The second solution is the one proposed by the twin paradox: A later encounter between both frames so that both twins can synchronize their watches. However, an encounter implies that at least one of both frames is ceasing its linear movement, in order to achieve a roundtrip. In the example, the spaceship will return to Earth (it is less probable that Earth will return to the spaceship).
A: 
I know the theory says that it's time which slows down 

No; that's a frustratingly silly formulation which unfortunately has been spread in some popularizations. What can be said with confidence is instead:


*

*In order to make statements about values of some quantity (especially whether they are equal, or unequal) the quantity under consideration must be defined; thereby also declaring how to measure its values, at least in principle.   


That's called specifying the theory; so it's certainly sensible to say that a particular theory implies a certain range of values of some particular quantity, or some particular relation between values of quantities evaluated in the same trial. Regarding the theory of relativity, and specificly its method how to measure (compare) durations it can be inferred (for instance):


*If two participants departed from each each other, and subsequently they met each other again, then the duration of one participant from departure until reunion is not necessarily equal to the duration of the other participant from departure until reunion; the ratio depends on the details of the (separate) motions of these participants (as described jointly, in relation to each other, by members of any suitable reference system).


The prescription of the typical "twin paradox" setup is such that the duration of one twin from their departure until their reunion is different from the duration of the other twin from their departure until their reunion.
As an illustration and general guideline: 
the duration, from departure until reunion, of a twin who was and remained a member of one particular inertial system throughout the trial (and who might therefore be called the "stay-at-home twin", or the "settled twin") is always larger than the duration, from departure until reunion, of a twin who was not (but who "changed between inertial systems", or who not been a member of an inertial system at any instant at all, and who might therefore be called the "roaming twin"). The ratio
"duration of the roamer / duration of the settler"
is of course (piecewise, schematically) represented by the number $\sqrt{1 - (v/c)^2}$, where the speed $v$ is determined of the roaming twin with respect to the members of the inertial frame to which the settled twin belonged throughout.

but my understanding (it may be wrong) is that time is just a human perception, it isn't a real physical entity.

Well, the word "time" is used in various more or less well-defined senses. To be unambiguous, The corresponding physical geometric-kinematic quantity whose measurement is defined in the context of the relativity theory can be called "duration". (Those some who are apparently less careful about their choice of terminology might call this quantity instead "proper time".) 

Can we infer from the Twins Paradox that the ageing process (cell decay or other biological processes) depends on motion?

The "twin paradox" illustrates the necessity and possibility to compare durations in the first place. Thereby we obtain the means to determine and to compare rates of various processes; such as comparing the rate of the ageing process of one twin to the rate of the ageing process of the other twin. (Perhaps, specifying the participants as twins comes with the expectation that their separate rates of ageing ought to remain equal, while they were separated from each other. But of course this doesn't spare actually measuring/comparing their ageing rates, trial by trial; and this requires a declaration how to measure, as indicated above.)   
