AAI Age categories calculated from 31st December in the year of competition.

Age group u9 Born 2016
Age group u10 Born 2015
Age group u11 Born 2014
Age group u12 Born 2013
Age group u13 Born 2012
Age group u14 Born 2011
Age group u15 Born 2010
Age group u16 Born 2009
Age group u17 Born 2008
Age group u18 Born 2007
Age group u19 Born 2006

Junior is u20, but you can also compete in this age group if you are u17, u18 or u19.
So in 2024 to compete in Junior age group for clubs competitions you need to be born  2005 or 2006 or 2007, or 2008
Senior Athletes must be 16 years or over on 31 December in the year of Competition, so u17 or older with no upper limit. Only restriction is that for men’s shot, discus, hammer and weight for distance, you have to be 18 or over on the day of the competition.
Under 23 in 2024 is athletes born in 2004 or 2003 or 2001
Masters is over 35 on the day of competition and you move up an age band on the birthday.