The Age Group your child will play in is determined by how old they turn in 2019, irrespective of when their birthday is during the year. For example:
Date of Birth 21/05/2006 - Turns 14 during 2020 - U14 Age Group
Date of Birth 02/01/2004 - Turns 16 during 2020 - U16 Age Group
Date of Birth 16/12/2010 - Turns 9 during 2020 - U10 Age Group