There is something I do not understand and its bothering me. If I have a twin and we are both in our 20's. Now let's assume I take a trip to space close to the speed of light, when I will come back I will be younger according to my twin (from the perspective of my twin in earth), but will I myself feel like an old man? I mean, will my biology be still as (old man as we know it) even that my trip was really short (according to my clock).
I hope I was clear enough in what I actually not understand here.