so i've been going off of the calorie calculator on the site that has it for people under 21 and have been eating about 2800 calories a day and have been perfectly fine with that...but according to the cc calculator (which from my understanding isn't really accurate until you're 21 anyway) i should only be eating somewhere between 2000-2300 calories a day. will my calorie requirements really change by that much pretty much right when i turn 21? or should the fact that i'm really active pretty much keep it where it is for a while?
If you are in maintenance and have some way of verifying this, such as an occasional weigh-in or even a daily weigh-in, if appropriate--then I would think you would continue to consume the 2800 calories, unless that leads to unhealthy weight gain. No, your calorie requirements definitely will not change "overnight." As long as you are active and not gaining weight, I don't see why you would cut the calories you consume.
No it won't change over night, but almost everyone finds at some point in their early 20s they are suddenly a bit heavier even though they haven't changed anything. Quite simply if your jeans get a bit tight don't carry on eating 2800 until then you're probably fine.
The science behind it is up until now you are changing and growing quite a lot on the inside long after you stop growing in height. When ever this stops you won't need the extra calories this uses.
From my experience, it happened to me around 24-25. Nothing really changed (in fact, my diet drastically improved from my teens to my mid-twenties), but over the course of two or three years, I put on about 15 lbs. Like vwiggins said, you'll probably notice that pants that have always fit are now uncomfortable. It's certainly no disaster - you just have to pay more attention and stop eating when you're just full, rather than blindly eating the same quantities you had before. Staying active is of course also a good idea - probably some of those pounds I put on had to do with having a full-time job and thus being desk-bound for a large part of the day. Even though I continued to exercise, some of the unintentional exercise I'd gotten as a teenager just through walking to classes and, well, being a teen, stopped all of a sudden. I've since adjusted my formal work-outs to compensate.
Ignore the calculator and go by what you maintain at. If you're maintaining at 2800, then you need 2800... but be vigilant for changes, because your metabolism will slow down eventually and then you will have to exercise more or cut back on intake to avoid gaining.
Calculators are not perfect. Similar calculations turned up numbers much too high for me when I was younger because they didn't account for the fact that my growth stopped at 12 and development stopped at about 13.
I'm 29 and haven't hit a slowdown yet - I burn more now than I did ten years ago. But that's probably because some medical problems have been treated and I'm more active.
Calculators are good for guidelines. Your stats change every day, either up or down a little bit. From the teenage years to the early 20s, most will slowly start to expend fewer calories for the same amount of activity.