"Do well with it" and "shoot it at my maximum capability" are two different things.
Most of us who come here can grab just about any rifle or handgun and probably hit a target easily enough and impress the average person.
But that's not in the same league as really achieving (or striving for) mastery of a weapon. My dad used to say, "well, you'd have got your deer," after just about any shot that hit the black part of the target. And that's how a lot of shooters traditionally have looked at firearms.
However, if you ever open the door to competition you'll discover that "doing well" kind of sucks, and "you'd have got your deer" just makes you embarrassed and mad. And that's to say nothing of introducing a timer into the situation and discovering that your "doing well" just went to "doing NOT well" when you were faced with time pressure, or that you were "doing well" but last in the group because it took you several seconds longer than the rest to make shots.
With all that in mind, Tirod's statements become a lot clearer. If you're picking up any of five, ten, or a hundred random rifles and pistols when you head out to the range, you might please yourself with doing "well" with any of them, but you'll never really hit a fraction of your potential that way. (Unless mediocre is your highest level of potential!) Going out day after day to train with the same firearm makes that firearm an extension of your body. You don't think, "where is the mag release?" You don't think, "ooh, keep that finger off the slide stop!" You see target arrays and put shots on them without being directly conscious of stopping to change mags or find the right stance.
I usually have found that I take several thousand rounds to really get completely in tune with a competition pistol when I change platforms.
I mention competition because that's where most people start to understand this. But the same truly applies to defensive shooting and training -- and even hunting. The consequences of missing a deer because you were a little clumsy with a rifle you don't shoot often are pretty minor. The consequences of not knowing
precisely where a shot will land with this pistol are a bit more severe in competition. The consequences of bumbling with an unfamiliar sidearm when you're faced with an armed criminal could be quite bad.
I wouldn't say it's a matter of just being young, but rather a matter of not having challenged yourself to focus and excel in this area.
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And that's not to denigrate anyone else. Some folks really enjoy having a collection of guns. That's their goal and that's their way of excelling: possessing a large and interesting quantity of items. And taking them out and having fun sending lead down range. More power to them!