Yes, the law varies from state to state, and the laws on long guns are more liberal than on handguns. Several states only regulate the minimum age to PURCHASE a firearm, not possess one. The most common minimum age to purchase is 18 for a long gun, and 21 for a handgun. The federal government also sets minimum ages for purchase.
However, for a school paper, your son shouldn't be taking the word of someone on a message board. The NRA has a summary of the laws, and so does the website of the Legal Community Against Violence. Have him take a look.
Just FYI, where I grew up in Louisiana, the traditional gift for a country boy's 12th birthday was his first personal rifle; normally a .22 That is still a very common tradition in areas where boys start hunting with their older male relatives as soon as they are old enough to pass a firearms safety course, and big enough stay on their feet when the rifle recoils.