What is the best dirt bike for a thirteen year old?

A second hand four-stroke trail bike, no more than 200cc like a Honda XR200 is probably a good option, the bike isn't aggressive or overly powerful, but still more than capable of doing almost anything that's thrown at it while being of legendary build quality and reliability.

The main deciding factor is probably how tall the kid is, as most of these bikes are fairly tall and even a six footer can find it a little bit awkward to swing a leg over the seat.

Ask around your local area, there's usually always a couple of old dirt bike tucked away in the corner of sheds that may need little or no attention to get up and running and are perfect for learning on.