It didn't have to be an AR-15. It could have been handguns, or a machete, or running a rented van into a sporting event, or any of a number of other things. Someone bent on causing a lot of casualties will find a way to do it, regardless of what laws are in place. The act of committing murder itself says the person obviously does not believe in the law. Age has very little to do with maturity overall. Picking an arbitrary age for 'responsibility' is really just blowing smoke. There are 12 year olds out there that I would trust to be responsible with firearms unsupervised, and there are 40 year olds that I wouldn't. A person't age is really irrelevant.

In this individual's case, he obviously had some mental issues going on. He was reported for it multiple times with no action taken. He tried to get help himself and no effective action was taken (a common theme with mass shooters). Blaming the tool used is just misplaced anger, I think.