Honestly if I exercised and ate perfectly from 20-29 I would've had a miserable 20s. Doing high-discipline stuff like that is way easier when you have a material reason (i.e. you've actually felt the consequences of not being fit or healthy) compared to when people just tell you "do it! you'll really thank yourself for it 20 years later!". Being in perfect physical condition at the cost of the psychological stress of forcing myself to do stuff I don't want to do every day does not sound worth it.