I did college at 18 and I was straight as an arrow, didn't drink, didn't party. I went back for another bachelor's degree at 25 and drank like a fish and got hooked on dope. Wrong people, wrong situations. Actually part of the problem was that I was 25. I didn't really have much in common with the 18-20 y/o kids I was in class with. Which meant that the rather motley crew I started hanging out with who were my age, rather than being Serious Non-Traditional Students, were a ragtag group of people who had drank enough to fail their classes and spend 6 or 7 years in school, a couple who had spent some time in either the army or prison and never completely lost the hard-living style they picked up in those places, etc. It was a high price to pay but I learned that you must be vigilant of who you are spending time with. School is no ivory tower.
That said, all it takes is staying out of the bars and away from anyone who could potentially drag you down, and it looks like you've figured out how to do that.
Good luck!