30 is considered an old timer in professional motocross. There are a few exceptions though, as long as you stay healthy (injury free) and are competitive.
Probably the biggest problem is injury
For professional motocross, the biggest bike that is allowed to be raced is a 450 four-stroke. For local race purposes there is a Vet class that allows any size bike to be raced (usually up to 525 since that's the biggest motocross bike that KTM makes).
there are many here are some: supercross arena cross motocross freestyle motocross indoor motocross outdoor motocross
Motocross, Supercross, Arenacross, Freestyle Motocross, Supermoto
Motocross, Supercross, Arenacross, Freestyle Motocross, Supermoto
Death, many times.
motocross tracks
I never got a serious injury from riding motocross, the problem is, everything comes later. For example you don't know how to stand on the bike properly, and you mess up your knees - that will introduce you to the world of pain couple years later.
A cr250 is a motocross bike
Some one died in football (soccer)
there is no "best" track...red bud has the biggest jump, but glen Helen is usually the most popular due to its location in California