I think that is fine.
I am 20 myself, and I teach for free to beginners at the grappling club at my school. When I was in high school, I taught classes at a martial arts school. I taught both young kids and older adults.
I had no problem teaching 30-40 year old adults even when I was 15 or so. They gave me their full respect because I knew what I was doing. They came to class to learn, and I was able to teach them well. I taught in a more "friendly suggestion" way than a "do what I say" way, but I do that anyway with anyone who is not a kid.
As long as the instructor is capable, age does not really matter.