as at, on 1st Jan of the cyclo-cross season determines catergory for whole season.
From 1st Sept 2007 for 12 months, age definitions are:
U10 born in 1998 or later
U12 born in 1996 or later
Youth born in 1992 to 1995
Youth Under 14 born in 1994 or 1995
Junior born 1990 or 1991
Senior born 1989 or earlier
U23 born 1986 to 1989
Women born 1991 or earlier
Veteran born 1967 or earlier

Juvenile - at least 12 years old or be in their 13th or 14th year
Youth - in 15th and 16th year 
Junior - in 17th and 18th year 
Senior / Expert / Elite - When a rider reaches his or her 19th year then the 
Senior or Sport category is the appropriate aged based category until they are in their 29th year. 

Gorrick Categories:

Under 12 - from 10th birthday to day preceding 12th birthday. 
Juvenile - from 12th birthday to 31 Dec of the year of 14th birthday. 
Youth - from 1 Jan of the year of 15th birthday to 31 Dec of the year of 16th birthday. 
Super Master - from Jan1 of the year of 30th Birthday to 31 Dec of the year of 49th Birthday. 
Master - from 1 Jan of the year of 30th birthday to 31 Dec of the year of 39th birthday. 
Veteran - from 1 Jan of the year of 40th birthday to 31 Dec of the year of 49th birthday. 
Master & Veteran Women - from 1 Jan of the year of 30th birthday onwards. 
Grand Vet - from 1 Jan of the year of 50th birthday onwards. 

Ability Categories: Fun, Open, Sport & Elite/Expert - from 1 Jan of the year of 17th birthday onwards. 
Beginners - from 12th birthday onwards. 

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