✿.*.
I think 3 is a perfect age to start. As with anything, SAFETY FIRST...use a bike helmet until he can maintain his balance. Also, teach him to slightly hunch forward, and put his hands on his knees when he loses balance or on the verge of falling.
My son started at almost 4, I signed him up for hockey tots and he did great. He's a bit of a safety monger and didn't skate that fast, which was fine with me!