Regarding this:
"People do not automatically become an adult the instant they turn 18 except in a legal sense."
Then at what age do they become adults? 21? 34? 40?
If 18-year-olds aren't mature enough to handle a firearm, then it's our society and parenting skills that need serious examination, not our gun laws.
In a legal sense, we have in the USA, there are at least three levels of recognized adulthood that I recall:
At 18, the individual is recognized as an adult in most aspects, excepting alcohol consumption, handgun purchases, etc.
At 21, full adulthood is generally recognized with a notable exception.
At 35, the individual could become President, regardless of their manners and how they carry themselves, if enough people vote for them.
So the law is an imperfect tool in this regard, though it generally works well enough in spite of some notable failures, that there can be a functioning society. We have to hold individuals accountable by these methods of measuring adulthood, imperfect as it may be.
Actual biological adulthood varies with the individual, but is probably less relevant than mental and emotional maturity as well as the attainment of demonstrable responsibility. Thus, a well trained 18 year old is often more competent than an untrained 35 year old.
But on gun control laws, we have always had them; the chiefest of which is the Second Amendment. All others should support and enhance it, rather than contradict it. We do need to have constructive discussions about these things so that something good can come out of these difficult days.