Mine played up as a 10 year old on a 12 year old travel team. He was lucky to have a great coach that knew how to use him. He eventually played as a 12 on that team. The coach took that travel team through 14 years old with great success.
He then played varsity as a freshman and the 10 year old experience proved to be very valuable indeed.
While in HS he played summer travel with his age group at 15 and 16. At 17 he played with an outstanding national 18 year old travel team.
The opportunity to play up at 10 and in HS led to leaving HS early and playing at one of the nation's finest program's as a freshman.
Playing up is imo, good for every above average player (assuming the coaching, expectation's and talent level is taken into consideration)and I think it is a huge plus for the outstanding player's....as long as it is "fun" in thier mind.