I hope I don't have a kid who wants to buy a bike at 18! I won't be able to stop him (or her!) but I'll want to! I hate mbikes (road bikes I mean), I just know too many people who have had nasty accidents.

In answer to your question FJ, I agree that 6 is too young. I would be thinking more along the lines of teenage years, maybe 12? Is there an actual legal limit? Maybe find out what it is, and present that to DH as the age you're happy with