Hi Point 9mm Carbine?

Im wanting to buy a hi point 9mm carbine, ive researched it a bit and decided that it would be perfect for my first weapon. I just have a couple of questions. First, my cousin said that its an assault rifle, not just a rifle. I know the age restrictions for purchasing rifles is 18, is it different for assault rifles? And is this even an assault rifle? i dont think it is based on older models but im not sure. Secondly, im worried about getting ammo for it. According to what ive read, you cannot purchase handgun ammunition if your under 21, and 9mm rounds are generally for handguns, so would I encounter any problems buying ammo? your help would be greatly appreciated.