C2

This is what I bought for my 12 and 14-year olds to use. It's enough to get them going and not too crazy fast. Still need to wear gear. They weigh 105-125lbs, and it's enough for them to get up the steep hill next to our house. I'm 180lbs, and I can ride it on flats, moderate hills, but it won't climb the steep hill with me on it.

Ninebot Zing (aka C series) are the only ones it makes sense to buy for kids.