Eh, sometimes there is overlap.
In grad school, I had a friend who at the age of 21, was told "I'm sorry dear, you have to be at least 14 to volunteer at the library". And she wasn't medically unusual, just 5'0 or maybe even 5'2, and somehow looking youngish.
In the same period, my advisor's daughter regularly looked 4 years older than her age, to people. When she was 11 people guessed she was 15, when she was 13 people thought she was 17. Maybe 5'6, dressing more 'maturely' with European influence. Darker coloration, not sure if that affects people's perceptions.