Men: Looks usually best between 20 - 35, but some may also look best between 25 - 39.
Women: Looks usually best between 20 - 35. If they're a celebrity, they can look their best longer.
This is just generally speaking. Ethnicity also plays a role.
Most Helpful Girl
Women look better starting from late teenage (15-18) and keep looking better and better. Once she's in her late 20's, little signs of ageing start appearing. They aren't noticeable until another decade but it does start with the complexion becoming more dull. However women who take care of their skin from an early age look good for longer, that's why celebrities look good for longer as well. I remember a Bollywood actress writing on a post "I have more ingredients in my face pack than my food."
Men, I don't know. But general observation is that they start looking good from 18-20 and look good even till their 30's, 40's and even 50's. However this varies a LOT because of work. Those who take a lot of stress start looking aged from their late 20's already but the ones who live a stress-free, organized life they remain youthful for the longest time. I have a maths teacher who I thought must be very young, but I was shocked to know he's 37 cause he looks one decade younger. He never gets angry and is always calm and cool. But another teacher in school who is quite young always shouts at his students and has a hightened attitude all the time, and now he looks older than most of our senior teachers. I think stress increases the ageing process in a man more severely than in a woman.
Most Helpful Guy
Women: 18-25, SOME women 18-35