They attempted transmutation when they were around 10-11
In the series they are 14 and 15 respectively.
So 3-4 years have passed since Alphonse was bound to the armor.
During this time he neither eats nor sleeps.
Edward sleeps and Alphonse uses this time to read, study, contemplate - whatever.
Edward would logically sleep from 7-9 hrs.
If we take worst case scenario - Edward sleeps 7 hrs and 3 years have passed since the transmutation.
365 * 3 * 7 = 7665 hours or 319 days that Alphonse has spent awake.
Realistically, though, I'd say Ed sleeps 8 hours.
That is 365 * 3 * 8 = 8760 or 365 days or 1 year.
Thus Alphonse is AT LEAST the same age (mentally) as Edward.
If Alphonse was a walking armor for more than 3 years (which I think is the case, but can't be sure since their age was never mentioned when they transmuted), he's older than Edward.
You could say that Edward is still older since he saw beyond the gate, but...
