I'd be of the "you're only as old (or young) as you feel". So for me, I don't think it matters too much, at the end of the day you might be 18, 21, 35 or even 60 on the outside. But we're all 1, 2, 3 or 4 or 5 or 6 or whatever on the in. Theres realism, which might not be the best word, then theres realistic.
Sorry, thats loads of numbers.
I can see what you mean though. Sometimes it can be strange to imagine or have your parent figure be your real age equal.