Always play at the highest level at your DD's age, not just with older.

At say the UTrip and local level, the level of competition, fundamentals, and play goes down with age after 14s - as the kids that are playing at a high level no longer play there.

If you want to move a kid that is the best in the ballpark at 10U to play on a 12U team, that's fine, but I would say better to put them on a better 10U team. Plus, let kid s be kids with kids their own age