It doesn't matter what the gender actually. Depend on each person. But you is still you even when you're in your 70. Your personality doesn't magically change when you got older. You still have your inner child it just you have more control on what you let other people see
And i dont really understand what makes it hard for you to believe he's 40??
perhaps that was just his true "face" that resurfaced, true.
however, don't they say wisdom came with age?
I'm not saying that you lost your childishness as you get older, look... even my parents are crazier than me when joking with their peers, but...
ah... might be better if I compared him with Jii-sama from "Jii-sama ga iku"? there's calmness that envelop his shenanigans and how he expressed his thought
sure, no matter how old a man is, (most of them are) on the inside
still a naive and mischievous child.
but how dp I say....