I started putting my youngest daughter in front of me and taking her for rides when she was about 2. My grand son took his first ride at about one and a half, on his grandmas bike with me. I let them ride in front on a 'sit up' bike until they can reach the passenger pegs then they get to go out on the sporty bike. Bottom line is whatever you and the kid are comfortable with and the laws in your state are. Your kids will remember the experience forever.
