I got my CFI when I was 19 (23 now) and probably looked even younger.
When I was instructing full time, I only remember having one student have a problem with my age- and he didn't come out and say it, it was evident in his attitude.
I sat him down and told him I was his instructor and asked if he had any concerns. He conceded that it was awkward having an instructor half his age. I politely reviewed his options with him, which were: he could either attempt to teach himself to fly IFR, get a new instructor, or continue with me and get over it. He got over it.
It's all in your attitude. Be professional, dress professional, act professional, and KNOW YOUR STUFF.
And for the love of god Chris Ford, don't debate with your students like you do on here.
It's only a problem if you make it one. If older students don't want to fly with you because of your age, reassign them to the worst (older) instructor at your school, and go recruit some new students.