haven't read the other responses but I'll tell you my experience...
first, I hate motorcycles. I've had a friend die and a uncle forever changed b/c of accidents. when i was married, I told my husband that he could NOT get a motorcycle.
Then we got divorced. Within a few months of being divorced, he had a chopper. The idiot rides without a helmet, but thankfully has a huge insurance policy that names me as the beneficiary.
Then my ex decided he wanted to teach our son, who is 7, how to ride a small 50 cc dirt bike. The response of "over my dead body" wouldn't have worked because 1) it's within his legal rights as a parent 2) it would make him determined to do it more. So I replied with, "only if he has the top of the line protective gear on at all times." That was already a part of Ex's plan.
It's now 7 months later and I dislike motorcycles just as much as I did before. Oddly enough, my son is REALLY good. I shouldn't be surprised since he got rid of his training wheels at 3. Him and his dad go out to the track every other Sunday where my son rides on a small, kiddo only, track. I've seen him ride twice. Not because I agree but because I love my son and I support his love for riding.
FYI... He is better protected than when I took him skiing this past winter. Not saying he won't get hurt, but I can't keep him from the things he loves in case he might get hurt. I would have to cancel skiing if that were the case. And rock climbing, and cub scout camp, etc, etc, etc.
Anyway, that's my experience.