I don't know where that 17 year old came up with the AR-15 he used to defend himself with when he was attacked by ANTIFA & Friends, but when the civil lawsuits start, you can bet the farm that the owner or previous owner is going to be named in the suit, and legal or not, right or wrong, they are going to go bankrupt defending themselves.
And by all indications that CHILD was pretty well trained in the use of firearms.
The question is, if you have a parent or parents, or even Grandparents, why don't you approach them to purchase one for you as a gift, which is legal IMHO but IANAL
I think part of the problem there is a state law that does make it illegal for a minor to have a rifle. In NH no such law exists.