Hello and welcome, really depends on how much you will grow, costumes are made to your size and some can be adjusted but it is some work. There is a group for under 18's and it's called the Galactic Academy, http://www.galactic-academy.net/. Now although they don't organise events some garrison's, squads, outposts allow GA members to attend some of their events (normally if an adult is present) but you would have to check with your local garrison, you can unit listing info here http://www.501st.com/members/displayUnits.php#garrisons
No issues with you being on this or other communities.
Good luck