Probably most of it during puberty, though it can still gain density in some parts until it eventually can start to fade, in some parts, or not, depending mostly on genetics.

When I get out of the shower it’s like trying to dry a wet towel with a towel.

I make my dog shake a couple times before getting out of the shower for this very reason, she finds it much more amusing than me scrunching her like a sponge.