Need Advice Regarding Content on a story

I am writing a story that is based around human mutants called Stretchans. They have the ability to inflate as a defense and a cooling mechanism. But I wanted the mutant race to be as logical as possible. So I indicated what age they are when they are either teenagers, adults, middle-aged, and old-aged. They are kids until 10 and become teenagers at ages 11-12, they become adults at 16, they become middle-aged at 35-40, and old-aged at 50.

A weird fact I made about them is that the only hair they grow is on top of their head and it's even a little elastic. Another weird fact I made about Stretchans is that no matter what age they are when they become adults, they always look young even at 60 years old they still resemble young adults.

 So, is it okay to have a character that is a stretchan be 16 yeas old or is it better to just call them an adult?