Well, if the end game is to get more CCW permit holders into the Universities with a future endeavor to try and get the feds to lower the age to buy a handgun, then I can see this as a worthy effort. However, if this has the effect of slowing or hampering a National Concealed Carry law, then it's not a good idea.
Lowering the age to buy a handgun to 19 nationally seems like mountain climbing. I suppose it should start somewhere, though.
Also, if this is designed as an incentive to get young conservative military personnel to become Arizona residents, that makes sense. It's actually quite clever if the Republicans are doing that. However, leaving only an avenue of gift giving and gun shows to acquire a handgun doesn't paint the best picture. Trying to find a 21 year old to gift you a firearm doesn't seem like a good idea. Gun shows work but I don't think most Arizonans even know you can sell a handgun to an 18 year old. And I wouldn't want to get caught up in a straw purchase.