Although my kids started shooting rimfire at a slightly older age I decided to go with the Mossburg Plinkster. The cricket stock size was going to be outgrown too fast. With the plinkster it is youth sized but still large enough for them to use through early teens adequately. In fact I have used it myself and while it is short it is not too short for an adult. It is NOT a tack driver by any means but for the price and design I have been satisfied.
I introduced the kids in stages based on their skill level, maturity and muzzle discipline according (roughly) to the following age table.
5-6 : BB long gun.
7-8 : Gamo Delta 500fps pellet. (hones their skills)
8 -10: 22 Rifle and pellet handgun.
11 and up: 22 Handgun, almost any rifle, shotgun.
at 12 (and with close supervision) they are permitted to choose just about any firearm in the safe. Subject of course to their size and skills to handle the gun.
I have no doubt that others may agree or disagree with this model because it is very subjective based on knowing our kids.
So that is my two cents worth.
-Rick