Start them as soon as they want! My kids LOVE shooting with daddy, and I gotta say it's one of my greatest joys sharing traditional archery with them. They are so receptive to learning. One thing I learned from others when I was starting with my kids...KEEP IT FUN, and they'll keep coming back!
My son Eli's first bow, at two years of age
His second bow at about three. Since these pics were taken I've cut him down some "custom" POC arrows (daddy loves this stuff).
By next summer I'll be looking to move him up to a newer, nicer bow. At the really young ages, the plastic and fiberglass is just fine.
His sister Gwen has shown alot of interest and at age six got a St. Joe River bow. David and Tracy are just amazing and really care about the kids they build bows for. Gwen's growing into this bow quickly! These two pics are about six months apart.