3 years old is fine. Start out on light days it will be more comfortable for the mom uh I mean the kid. As everyone gets more comfortable on the boat then increase the wind. Not to be confused with breaking wind... Don't make the mistake of putting them in a harness and letting them hang out on the trap wire. My son (and girls) want to put trapeeze wires on my Stiletto now.

Start early, makes for excellent crew and more time on the water.
Good luck,
Clayton
H16, S27