Haha, well, I actually JUST turned 20 about a month ago, so I may not be entirely relate-able, but I've definitely been over 300 pounds. For the second time in my life, I've gotten out of them; first time was three years ago--junior year of HS--then I lost about 35 pounds, and eventually gained it all back and then some (ugh). But I've made a commitment now to permanently stay under 300 pounds...I never want to see a 3 as the first number on the scale ever again!
You've got time, and you're still young! Keep it up.