I dunno, probably 6 or 7, maybe 5.

I had training wheels, but my parents decided to help me learn to ride, so they had me ride without the wheels while my father helped me balance by holding the back of my seat. I was pedaling, balancing, and my father was balancing me from behind, and they were saying, good, good, keep pedaling, then I noticed their voices were farther behind, and I looked back, and they were standing back there watching me ride by myself...