Start with a CO2 air pistol first. If it balks at that, you may have issues. If it is cool with the air pistol, use a .22 revolver with shorts in it. Then graduate to .22 LR and then if all that works, you should be able to graduate to the .45's, .44's or .38's. It's typically easier to start younger, but nothing says they have to be young to get accustom to it.