My son is almost seven years old and still can't ride a bike. He loves to be outside and play, and he wants to ride a bike. His bike has training wheels, but the moment I take them off and try to help him ride, he's not even close to exerting any kind of balance-- he's basically just falling over and crashing every time.
This past weekend we kept going down a slight hill and he just doesn't react to falling over (other than to keep falling). For those suggesting taking the pedals off-- he has the same problem keeping balance on a scooter. The gyro-wheel idea is cool, but I'm going to keep trying this downhill thing everyday for a while before spending the money on that wheel.