They don't (up to a certain age). People just have times when they look better or worse. I am 22 now but I can safely say that I look better than I did in my teens (better fashion sense, healthier eating habits, working out).
I feel like guys in their 20's haven't grown into their looks yet. It really depends though. People tend to be more established in their 30s so they can afford to be better looking in their 30's and 40's vs their 20's and teen years.
Talking about beauty, guys peak around the age 19-23, I think. Everything else (body structure, maturity) peaks around the age 25-28. Dunno about gals.
Of course it varies person to person, but in general those are the ranges I would say. Basically your 20s. Anyone saying under 18 is kidding themselves, lol, that's just gross. Some 18 year old girls look really hot (models) but they usually look even better in their early 20s. Guys, regardless of how built and/or mature you are, still look like boys before a certain age.