Is U16 the point where positions should become much more specialised?

U16 is a reasonable age for roles to become more specific, but I would not make specialisation a rule for every player. Some players are clearly benefiting from repeated detail in one position. Others are still learning because they can connect two or three related roles. The useful question is whether the variety is adding something or simply interrupting continuity. If your daughter is playing full-back, midfield and wide because coaches can see transferable strengths and give her clear feedback in each role, that can still be valuable. If she is moved because she is reliable and fills gaps, she may be getting less deliberate development than teammates. I would ask for a main role and a secondary role rather than one permanent label. That gives enough repetition for deeper learning while preserving some versatility. Older players need clarity. Clarity does not always mean only one position.