125 or 250 if you fit on the but but they run together in the 250 class
prefferably the 85cc class
85, supermini class, 125.
85 cc or 150 cc
****** IN MOTOCROSS AT THE AGE OF THIRTEEN THE CLASSES ARE **85 CC BEG **65/85 CC OPEN **** IN ATV OR QUADS**** ** 90 CC LOWER **
They would have to be in a 90 cc class or lower to race quads. But you can be any age to race dirt bikes.
if he is just starting then 250 beg
The following is from one of my local motocross tracks, most local tracks should have something similar: 0-85cc 7-15 Beginner 0-85cc 14-15 years 150cc and smaller 4 Stroke, 15 years old & younger
 and any of the novice or intermediate classes depending on their ability
Depends on height and weight.
25+ vet A and vet B or 40+
The following is from one of my local motocross tracks, most local tracks should have something similar: 0-85cc 7-15 Beginner 0-85cc 14-15 years 150cc and smaller 4 Stroke, 15 years old & younger
 and any of the novice or intermediate classes depending on their ability
I don't know that it is the most important race ever, but what stands out in my mind is the 1981 Motocross DES Nations where Team USA won completely unexpectedly and brought on a 13 year domination of the MX Des Nations by a country, up til then, considered to be inferior in motocross. daytona supercross is always the best race and atlanta 2009 supercross is the best race in history
ive been tring to get into motocross racing for a long time. next year hopeffully but it is very expensive. your looking at around $500 - $1000 to race for a season