Your league age is determined by the age of the athlete prior to May 1st.  For example if your athlete is 10 years old and their birthday is June 1st, they should be registered to play in the 10 year old division.  If your athlete is 10 years old and their birthday is February 1st, they would turn 11 before the age cut off date and would have to play in the 11/12 year old division.  Athletes are permitted to play up an age group but never down an age group.