There's a continuum - sometimes players and coaches will align positions to the kids strengths - best sticks play attack, best/fastest athletes play middie, biggest/slowest kids play defense, goalies... well they are just special. These attributes may change over time, positional training gets more focused in middle and high school but along the way kids should be able to move into different positions provided they've been working on all skills e.g. stickwork, dodging, etc.