a couple of them raced bikes and quads. there were six girls in the bike class and five or six in the quad class (several of them rode in the regular classes too). one of the quad ladies had a scary crash over a double. she was down on the track for probably ten minutes. they red flagged the race. she ended up getting back on and finishing the race.
my daughter is 10. another of the girls is 11 (she is pretty quick, finished third). the two "serious" racers of the bunch are about 18. those two were really fast. they are competitive in the regular classes. the girl sitting on the quad is really fast too. she can hang with the boys, no problem.
the reason most of the pic's are quads is that the lady taking the pic's was one of the bike riders. so she was on the track during the bike races.