I returned back to school at 23, however, I know lots of people that did when they were older. Something just clicked inside me and I realized that A: I could accomplish more with a college degree and B: I will take it much more serious now that I am older.
College was also the start of me slowly starting to moderate myself. Now, lot of stuff happened after I got out, but I am much better now. During college, I was often more then happy with just beer and bud. And when the end of the semester was coming up I would always get really serious and basically stop using anything.
Now, obviously my timing sucked as we hit a depression right when I got out, but I still got a job.
It did take me a long time to convince myself that I deserved good things and nice things to happen to me.