What do you believe to be the most dangerous time for a shooter to have an accident or do something careless? The first day? The first month? After a couple of years when comfort sets in? In the later years when the senses are not as sharp?
Complacency breed contempt. When a shooter has a 'near miss' (pun intended) on safety from not following the Four Rules, that is when they become dangerous. BTDT. You might get away with violating one of the rules and avoiding an tragedy; I did and luckily only got a hole in my pants and the floor as a result. Could have been a lot worse.
I was trying to holster my 1911 in an unfamiliar hoster to me, when it was loaded. This was before CCW in WI, and I wanted to get used to that holster, and carrying the full weight of the 1911 with it. In my 20/20 hindsight, I should have put the unloaded pistol in the holster, and then slid a mag in, thus (almost) duplicating the carry weight. The one round wouldn't have made a difference. For those interested in the technical details, It was a King Tuk holster and while sliding the gun in, I had bumped the thumb safety off, then was depressing the grip safety attempting to slide the gun down in. The full roundness of my hips pushed the leather out enough that the molded plastic hit the trigger, and kaboom. I now physically push the thumb safety up while holstering.
Obviously when a person untrained and unfamiliar with a gun, made worse by a small amount of knowledge, handles a gun, bad things can happen. I got lucky there, too, as a 12 year old. Put a hole through the kitchen floor, and luckily didn't hit any of the 7-8 people in the basement, because of my ignorance of how a DA revolver worked. I resolved then and there to cure that by learning everything I could about every firearm I could, and started by reading Ian V. Hogg's boo on how firearms work. Been expanding my knowledge ever since, and I am nowhere near knowing everything.
Form my experiences over the years of instructing kids (4-H) and soldiers, (same things really, except they all become instant experts upon enlisting) If you get the Four Rules drilled into their minds right off, and keep reminding them (in 4-H we went over the Four Rules before every range session, and you military guys know (at least Army) you have to do a class every time you go to the range) they are less likely to get complacent.
Great question OP. Not trying to muddy the waters, but I think the most dangerous time is continuous. I have only owned firearms for the past seven years, so I might still be in the conscientious zone (in my sixties now, so add in senior status too). Every time I handle them I realize, but am not fearful of, their potential to cause damage so I am very careful to avoid mishaps. However, no matter how aware or careful I am, I am also aware that I can make a mistake for whatever reason. Hope this makes sense.
Keep that mindframe, and you'll probably never have an incident.