I held my hair well into my 30s... then the minute i got serious about lifting and started training on schedule 4-5 days a week, boom, went thin fast. I'm sure there's some scientific connection there about DHT or t levels or whatever. most of the guys in my gym over 25 are shaved tho.
Not really . It is all about hair follicle genetics ( their response to DHT), pure and simple.
I got lucky and at 53 still have a full head of hair as did my dad and both grandpas.
I can't say for sure what I would do if it started to thin out.
BUT, the main thing is to control the things you can and live a healthy , fit lifestyle.