We rode with our son in a well padded trailer when he was about 9 months. He wore a helmet although there were many times that we would stop and the helmet would be in his hands. We had a couple of rides though where we were not on the same wavelength and he cried A LOT where you are just SO SURE that everyone is about to call the authorities on you!. We had him in a rack mounted bike seat by about 3. It would be so funny when he'd fall asleep with his head PINNED against your back! We're planning our first bike tour this summer - he'll be on the back of the tandem. Start 'em early but be safe and smart ;-
Tandemracer - your baby is adorable!