Well when I was in the 6th grade, I was experimenting with makeup one day and my mom came home and was so mad that I was not allowed to wear any for a month...then when that month was over I hit the Bonne Bell lip stuff like there was no tomorrow. They had one that was a marshmallow scent, a VERY frosty brown/silver, and I rocked that like there was no tomorrow (to my embarassment). That summer was when I hit Wet/Wild and Bonne Bell, and I had the WORST looks ever. My mum said that as long as I kept the eyes neutral, I was fine...but there are pictures of me back in the 7th grade (fall of 2000) at the ordination service for a relative, and I had on the most atrocious dark purple lipstick ever. It was from the Halloween collection. Gold lipstick? I had it. Silver? You betcha. Blue? So mine.
I guess you can say that I was doing it all, albeit terribly, by the time I was 15. There were a lot of days I would never do makeup, because I was more concerned with getting as much sleep as possible before school before having to leave the house at 7:04 AM to catch the bus that would be there by 7:11.
Back to the topic, I see no problem with people using it once they are in middle school. However, to avoid mistakes like mine, please a) show them the good quality makeup, and b) either give them a lesson, or better yet, take them to a beauty salon or a counter to get a lesson. It could be a birthday thing almost-get them a good quality set of makeup.