I talked to a recent graduate who said something along the lines that "compared to the past people are now running faster sooner" which has some merit to it.
I think back to HS track and cross country and appreciate the fact that you form a team with the kids you grew up around. A same class acquaintance's brother or sister, upperclassmen and underclassmen and that kid who you grew up down the street from but never really struck a common ground. Before you know it a 5-6 kid grab bag running consistently in the off-season can run under 16:40 for 5k. That's the beauty of it and you go out and represent your city, area of simply high school. When you form a good nucleus, have good leadership and have a go at it for a couple summers and winters it's really fun and engaging. And the focus for the hs elites is getting into a good college. Oregon, Stanford, Colorado, etc. I think there's something to be said for the majority of high schoolers having a large support group behind them. If not a bus of 80 kids standing behind their #1, it's the local news coverage, the teachers and parents knowing you as "that runner [guy or girl]". It's the horns that honk at you on training runs in the off-season. High school can be a very happy time with so much going on. Homecoming, soccer and football games, the band, etc.
When you move off to college it's not only a step up in training. It's also a step up in academic demand and responsibility. Once you get secured into a good school it can be a huge weight off your shoulders. You no longer have to worry about getting INTO the school. Unlike high school running college running is not just an extracurricular with your hometown grab bag. It's a chosen lifestyle and at most schools everyone there was somewhat of a standout which can take away from your drive. After all, you don't have to be there (you were already admitted). You don't have to perform (there's no next level to anticipate for 99.9% of the population). The field is much greater and filled with foreign stars. Between it all you also have to find an adequate balance of academics, athletics and being social so you don't get awkward. Sometimes people can't hang with the upped mileage. Sometimes salaries and potential romantic partners can take over your interest. College is definitely a different ball game.