Well, yeah. But there arnt any 16-18 year olds in college.
I think the reason why there is such a discrepancy is because young adults under 21 really never participate in true adult settings. They don't go to bars, conferences, or have serious jobs.
Once your 21, so many doors open to you. You're now able to hang out at a bar where the mean age is 26 or older still. These people arnt going to put up with your childish nonsense. If you have a real job, you're also working with mainly adults. Again, there not going to put up with you if you act like a 20 year old.
Personally, I am a night and day difference from who I was before I was 21. Dressed like a chump, not confident around people, didnt really care about my appearance. I used to let my hair get really long, to steal from Moe, until I had a 'jew fro' and then get a haircut. Pretty much your typical engineering dork.
On the other hand, I have "friends" I know from school that are nice people, but immature as all hell and 21-22. Then again, they are engineers so...
In conclusion, cut the freshman some slack. They're straight out of high school and around older people. I am sure 99% of them are humbled the first day when they realize there a nobody, whereas back in high school they at least had a group of friends.
My friend and I were walking to class and this guy ran up to us and asked us where the math building was. The poor guy was a total dope...

I just pointed and said 'over there'.
College is great. Try this trick. I do it every day when I'm walking on campus. Look, actually LOOK at the person that's about to walk buy you straight in the face. 99% of them will look down and walk past you because they don't have any personal skills. If the don't look down, smile at them. THEN they'll turn their head or look down.
Now look at any ADULT and smile at them. They will say hello to you. That's what I can't stand about 99% of the people in college, they don't know how to socially interact with strangers.
The best fun is when I see someone walk all CONFIDENT down the sidewalk. I'll look right at you Mr./Miss confident, and they ALWAYS look down as I walk past them.
That's why I tell people who make these dating advice threads not to listen to people who tell them 'just be confident, act like it and she'll believe you'... It's so painfully obvious to someone when you're not confident, and you'll get called out on it (at least you will if I see you faking it).
I like talking/interacting with people because after a while you can read them like a book before they even say anything. You can tell their mood, and if the conversation's going to stink or not. 9/10 its not that hard to tell off the bat.