our youth national teams can compete with almost anyone in the world until they turn 18.
The drop off is easy to explain. It is all because of college soccer, where the vast majority of elite players in America end up. I could write an entire book on how college soccer destroys the United State's ability to compete on the world stage, but I will boil it down to this: The NCAA only allows college soccer teams to compete for 3-4 months of the year (with spring exhibitions allowed). So from a sheer numbers perspective, while other countries make their 18 year olds pro players with salaries who play year round, we limit our elite players in the prime of their professional development to 3-4 months of high level training and games a year.
What happens as a result of this is that in their prime professional development age, American soccer players don't play by the same rules as the rest of the world. Whereas soccer at the top level is largely a players game, college soccer gives much more power to the coaches. As a result, our players don't learn how to think and read the game in real time because in college soccer, if something isn't going right, the coach can make a sub. If someone is tired, the coach can make a sub. This creates an unrealistic picture of the game at the college level which has much more to do with coaching decisions and physical exertion than it does with playing the nuances of the game. For example, the college game usually is high press, intense, and brutally physical because guys can essentially run their heads off and then get subbed off when they are tired, then come back on and do the same thing. This is completely unrealistic at the elite level when you have to conserve energy and think about what you are doing in real time. As a result, American soccer teams, even at the elite level, tend to be incredibly fit, strong, and organized, but we lose because we are very unoriginal, unintelligent (soccer wise), and technically not as good as the teams we play.