My son started on the tank of my '94 Sporty at 2. Just around the little private developement I live in. At 4 with a properly fitted street legal helmet and his feet resting ontop of the leather saddlebags he started riding on the back, everywhere we go.
5 years later my wife got her own license since she was getting left behind too much! LOL. Seriously, I had a problem with having to leave one of them behind. So she really did get her own license and now we ride together as a family all the time.
My son will be 14 in 3 months. He's ridden over 30000 miles with me. Including a 1000 mile father son road trip out to and around Ohio and back with no particular destination in mind, and then a few months later out to Milwaukee for the 105th fiasco. I hit a tire aligator which broke both of the rear swingarm mounts causing the bike to go into a wobble. Without being asked he calmly spotted lanes and for me so I could get to the shoulder safely. He also works on the bike with me. In the last 2 months he helped me change the mufflers, and do the alignment from the group buy alignment tool we bought. Actual help wrenching not just handing tools or standing around.
I don't know many parents who have that much interaction with a teenage child but I'm convinced it's helped our relationship and may even have kept him from getting involved in the wrong type of activites as he's part of our riding club and mostly participates in that stuff outside of swim team and scouts. The government and Ralph Nadar types may have an opinion on what age, but personally putting my son on my bike when I did was the best decicion I ever made as a parent.
Ride Safe,
J-Carr
P.S. My signature pic is he and I on our road trip to Ohio this past summer.