I'm adding the limiting factor "young" because first of all, I think older people are less guilty of acting this way. Presumably, they've learned it doesn't go over well, and in addition, they are more likely to actually BE an expert with a lengthy track record on a given subject, so their spouting off about whatever it is can be easier to swallow.
I generally define a "know-it-all" as someone who is not only a windbag (I'm a windbag on many occasions myself), but also has to generally be RIGHT all the time; who isn't open to alternate points of view; who feels a need to make nasty remarks smacking down someone else who might be a novice or might simply disagree; who constantly has to refer to their own experience; who is basically a self-promoter at the expense of discussion.
I don't understand why people seem to like and accept these folks, especially if, like I said, it's not a matter of "Oh well, that's just Joe Schmoe who has 25 years in the business and thinks he knows everything, chuckle behind his back and forget it."
I've run into these people in literally every single arena of interest to me for many years. Music, law, promotions, photography, film, writing, school admissions, family life, you name it there is some 22-year-old who thinks they know EVERYTHING about it. Years ago I reached the point where, if I actually know something about something, I tend to keep quiet because I have ZERO desire to be like these self-advertising blowhards in ANY way, shape or form. I just let them run on and say "Mmmhmm" a lot, unless they really annoy me (for example, by being super rude to someone else) in which case I'll sometimes tell them where to go. If I really want to have fun, I'll advocate the opposite position sometimes just to watch them blow a gasket. I usually figure they are either in love with the sound of their own voice, or hopelessly insecure.
Why does everybody but me seem to tolerate these people so much, hell, even LIKE them? There is nothing likable, or even interesting, about them, and given that they are young and acting this way already, I always dread that they will simply worsen with age. The saddest part is that they usually do have good knowledge that they could share or use to help others, but they aren't interested in helping or sharing. It's always just ME, ME, ME.