I bought my first handgun from my uncle at almost 20. This was in Michigan. The way the law was written here, at least at that time, was that, "a person over the age of 18 may apply for a permit to purchase a handgun, and may purchase a handgun." That's all that was said in the law back then. When compared to the federal law that one must be 21 or over to purchase a firearm from a federally licensed dealer, it left the loophole that an 18 year old can apply for the permit and buy a handgun, from a non-federally-licensed dealer, such as a private individual. I applied for the permit through Detroit, where I lived at the time, later picked up the permit, purchased the gun, went and registered it, and nobody said anything about it.
Compare the actual wording of state law with the federal law, and you might find a similar ciscumstance, but by now, most state laws were changed to cover this gap.