AGE CATEGORIES

According to USA Dance Rulebook §4.7.4 your age category depends on your birth year and not your actual age at the time of the MAC.
In other words, ignore your birthday and use the year you were born to determine your age category.
 

MAC Age Categories

Adult 19+ Born in 2000 or earlier
Senior I 35+ At least one partner was born in 1984 or earlier The second partner may have been born in 1989 or earlier
Senior II 45+ At least one partner was born in 1974 or earlier The second partner may have been born in 1979 or earlier
Senior III 55+ At least one partner was born in 1964 or earlier The second partner may have been born in 1969 or earlier
Senior IV 65+ At least one partner was born in 1954 or earlier The second partner may have been born in 1959 or earlier
Senior V 75+ At least one partner was born in 1944 or earlier The second partner may have been born in 1949 or earlier
 

Junior MAC Age Categories

Pre-Teen I Born in or after 2010
Pre-Teen II Born in 2008-2009
Junior I Born in 2006-2007
Junior II Born in 2004-2005
Youth Born in 2001-2003
Under 21 Born in 1999-2003
 

For Further Information…

The above years are provided only as guidelines. Consult the rules and tables at section 4.7.4 of the USA Dance Rulebook for full details.