I don’t see any links to Vsauce’s video on this so I’m going to assume every response is wrong. TLDR: Styles become associated with eras and people in those eras become associated with our perception of that age bracket.

Lower testosterone is probably a big part of it. Look at 23 or 24 year old enhanced bodybuilders. they look like theyre 35. For whatever reason peoples T levels have been going down about 1 percent per year for the last 50 years. Its bound to have an effect.