I'm a 13 year old training with adults and everything seems fine. Besides one or two other people who only come occasionally, the rest are over 21 from what I can tell. The only issues I really see is size, and that's only in SOME techniques.
And you do realise it's difficult to properly grasp the Sanshen No Kata from learning on an internet site. The angles and dynamics, particularly the final shuto in Sui, are rather subtle. It'd be difficult to learn from a website, and almost impossible without a partner. In one of our classes, Sensei had each person demonstrate the Sanshen No Kata without someone actually coming in with a punch. You could clearly see the problems to do with targeting, angling etc with most people when doing it without a partner.