I've been told to calculate the age of the Universe at $T = 1 \, \text{MeV}$, assuming that $a(t=0)=0$.
Now, I've already calculated the value of $H$ at that temperature, which was around $H(1\,\text{MeV}) \approx 0.6 \,\text{s}^{-1}$. I've also shown, that in a radiation dominated Universe, which I assume much be the case at $T = 1 \, \text{MeV}$, that:
$$H = \frac{1}{2}t^{-1}$$
So basically, my idea was just to solve for $t$ in that equation, and use the value for $H$ I calculated, and then I end up with $t = 0.85 \, \text{s}$, which seems okay reasonable to me, but, my main question is the info: "assuming that $a(t=0)=0$". I haven't really used that information here, so either it's just not important, or I have missed something. But what ?