I think ObiJohn
had a good thoughtful response and I have a 6 year old daughter so Im there. She has asked and I dont think she is ready. Each kid is different go with your gut and use your judgment of your children. My dad gave me a ride on the back of his bike when I was 4 or 5, I had a minibike when I was 6 and I was winning AMA flattrack races at age 14 against adults. Everybody is different.
"They're not worried about 'What if', they're more like, 'Hey, watch this!'"