Insurance regulations require that all drivers be at least 8 years of age to drive on the big track and around 48" tall because you can't drive if you can't reach the pedals.  We also have the Kids Track for 4 -7 year olds. Height has not been a factor for the kids track up to this point.
The kart manufacturer and insurance regulations also require that all drivers be under 300 pounds. We do weigh drivers to ensure they are of the appropriate weight.