Anecdotally I feel I noticed a very fast ageing speed between 38 and 40. Suddenly got white hairs, feel more tired, more wrinkles, way harder to keep VO2max up (I run a lot), muscle sores after training suddenly lasting up to 3 days instead of 1, face looks older, etc.
I feel like that all happened real fast around this age.
It's like there's two versions of me now, the one who was somehow moderately fit by biochemical decree, with a healthy amount of flesh to his face, voluminous dark blonde hair and a pleasant complexion...
... And the grey haired, weathered, lined, dessicated mummy I see in the mirror. I love my kids dearly but the constant caring really takes something out of you. That and the whole getting older thing in TFA.
I keep telling myself I'll get a gym membership soon to reclaim some of my dignity.