I got sober at 37. I think age and maturity have something to do with it - but as you know, some people are very "mature" for 18 and others are very immature for being 40.
In your 30's the scene with bars, parties, etc starts to dry up with old friends. They are moving on, careers, travel, families, etc. In my case, I ended up "keeping the party going" for many years too long. I had been drinking heavily for most of my years since 18. Your body starts to show signs. Give out. It's really, really hard to be an alcoholic and the wear and tear breaks many of us down. Also, at 37 I was not a kid anymore. Real life is in full swing. This is "it". Did I want to live through my 40's drinking at the same pace? Or was I ready for some life changes? I was mature enough to accept the fact that it was now or never. Oh yeah, it also took a few ambulance rides. It really came down to the basics: live clearly or die a slow, drunken death. I'm not sure at 18 or 20 I would have realized the choices were so clear-cut. I just turned 40 and have 3 years sober, so happy I made the choice to live in this fashion.
One thing we all say: we wished we'd quit sooner. Youth can indeed be wasted on the young!