There is not a specific height you have to be... it all depends on how comfortable you are on that bike.
I ride a RM 125 and i can hardly touch the ground, yet i race it. I would suggest starting out with as big of a bike as you can get.
-woody.eldridge@Yahoo.com
The 110.
go to a local dirt bike shop and explain the problem to them dirt bike stores always have mechanics to help
you would need about a 125cc 2 stroke
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.
None for real
maybe 60mph
a yz 85 because it has a big top end
kx 100 is faster on bottom end speed kx 85 is faster on top end speed so they are about equal to some extent. ;)
85 years old
arond 80-85 mph
About 85 90 mph
Around 80-85 pounds of torque.