Probably a yamaha ttr 110
Kawasaki Kx500, any year will do, but the best bike for a beginner is a Yamaha yz490
For a 13 year old it depends on your size and your experience with dirt bikes, but an 85 is usually the best size bike and fastest for a 13 year old. Unless you are really tall which you would need a 125.
It depends on your height and how you can control the bike. My first bike when i was eleven was a Honda CRF 150F.
a 120cc dirt bike is suitable for a 17 year old
an exercise bike would be the best start
Yes
no you have to be at least 14
Yamaha
No.
a xr 100
A Yamaha 125
yes just get them a 50cc or 70cc dirt bike