Started both my girls on dirt bikes at the age of 5, but both had been riding bicycles with no training wheels for close to a year before.
started riding them on my street bike around 7, belted to me for a year or so and kept them outfitted with the best ridding gear to be had.
Now they are both in college and have their own bikes ( both white FXDLR's) the oldest races Quads Semi-Pro and her sister and I do the wrenching & pit work.
The younger one has her eyes on a V-Rod Destroyer.