I take care of myself and don't smoke and only drink occasionally and People believe that I look younger than I am, people usually believe that I am 8 to 9 years younger than I am.
There are some students at my college that I attend who look older, some of them are 19 and they look like they are in their late 20s or even early 30s while other 18-22 years old look almost like high schoolers.
Some people gray in their 20 while others don't grey until their 50s. For example most of my family don't get grey hair until their late 50s. My maternal grandmother who passed away at 83 had gray hair not white hair due to nearly half of her hair was still dark and not white.
I even see people that have fine laugh lines at 18 so people age differently some genetics and some environment. Smoking and high sun exposure would age skin faster. Darker skin has higher melanin and more oil so darker skin tones age slower on average since melanin protects skin from UV light.
This is an example of sun aging skin over time, left side exposed to lots of sun and the right side had little sun exposure; man is a truck driver.