Time differential.Except we're talking about characters that we know what they looked like when they "died" and they don't look like that now.
EXCEPT.... that would have to include 11 and 10 together, so 10 would look "older", but from the photos from on-set, he's not in any aged make-up.
Wellll he could use his sonic. It's Doctor Who alright? The point is that the whole thing can be solved in one sentence and it only has to make sense emotionally and dramatically but certainly not nitpickingly.