I'm not sure there IS an age where you should start to pigeonhole players into single positions...it's easy to do because both the players and the coaches get "comfortable" with familiar players in familiar spots, but it's important to keep testing to find out what options are available, both for the individual player and the team. Finding out that a player can handle multiple positions opens up many more chances and strategies for team success, and it makes the individual player more "marketable" when moving on to other teams and situations. It's usually worthwhile to try to shake them out of their (and the coach's) comfort zone and see what they can do in other spots--even on u18 and high school teams, some of my best defenders have also been my best scorers and vice versa, and being able to swap players between high-pressure and low-pressure spots during a game is a great way to rest them on the field without having to sub.