Where do you have to be 21 to buy a finished lower? Well, I could probably guess a few states. Here in AL 18 is legal.
For AZ, go you your state website and read all the pertinent laws for firearms. If after reading the laws you are still unsure, then find competent help. What you don't want to do is wind up in trouble with BATFE. You could wind up in real trouble then. Several years ago, a young man who worked at a hunting store in Pensacola FL responded to an ad selling an AR rifle. When he got to the house, a lady was selling her husbands gun as he had died. He was former military. The rifle was actually an M16. He recognized this but bought the rifle for what he thought was a steal, figuring he could make good money by re-selling it. He took it to a public range and was firing full auto. Someone called BATFE, he paid a large fine and can longer own any modern firearms. He can own a muzzle loader. Too much at stake to guess at what you are doing. Education and knowledge is the key. Do your research, make certain you are legal. Do you KNOW the difference between an AR pistol and a short barreled rifle? One requires a tax stamp and if your build accidentally requires the stamp, without it you are just as much in violation as the guy with the M16.