Metabolism slows somewhat because you are no longer actively growing. Growth in females continues until about age 18, while it doesn’t stop in males until about age 25. And as people have pointed out, there are also a whole slew of lifestyle changes (including increased access to especially tasty food) which happen around the same time the metabolism shift from growth to maintenance is occurring. So I don’t think this is as big a mystery as you’re making it out to be. All it takes to gain very significant amounts of weight are small calorie imbalances over a long period of time, and that’s not hard to achieve.
I think the biggest single factor for me was going from a bicycle to a car. Ride my bike to see my friend twelve miles away, no problem. Ride out to the state park ten miles away, no problem.
After I got a car, I didn’t ride a bike for years.
Personally, I didn’t start drinking until I went to college. I found a new hobby and the pounds came with it. That, and like someone above said, easy access to a car instead if walking/biking.