While painfully annoying, potentially dangerous, and certainly rude, there is a flip side of the coin here...namely the assumption, however "natural" it may be, that any given person actually comprehends the nature of firearms and firearm safety they way they should.
As I learned a long time ago in the Navy, when you "assume" something, you make an "*ss" out of "u" and "me".
One would never intentionally give a firearm to a person of known inexperience (such as a child, young adult, or first time shooter) without proper instruction on safety and handling. Likewise, one should never give a firearm to a person of unproven experience without somehow either confirming their experience or reinforcing proper behavior in the process.
Much of what we all like to refer to as "common sense" has been relentlessly pounted into our heads over quite a bit of time in order to ingrain it into our every action with a firearm. This leads to the development of good habits which embody safe handling of a firearm. A person's age alone does not confer either "common sense" or "good habits".
I never forget that I am ultimately responsible for all my firearms regardless of whom I let handle them. Yes, there is an element of personal responsibility on the individual of the person handling them, but they are MINE and I always treat them as such.
Most cases I'm pretty sure of a person's experience before I hand them a (properly cleared) firearm. But I will still carefully observe everything.
When I'm uncertain, I might do something like clear the firearm, hand it to them, and direct them to verify it free and clear and to keep the weapon pointed in a certain direction. Observations and corrections follow as necessary.
There is a need to weigh further actions based on observations, of course, but I've found that in most cases people are not "maliciously ignorant". In fact, with some tutoring, a significant portion of potential "problem children" with respect to ignorance turned out to be very adept students on the matter.