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 2001 or earlier
Senior I 35+ At least one partner was born in 1985 or earlier The second partner may have been born in 1990 or earlier
Senior II 45+ At least one partner was born in 1975 or earlier The second partner may have been born in 1980 or earlier
Senior III 55+ At least one partner was born in 1965 or earlier The second partner may have been born in 1970 or earlier
Senior IV 65+ At least one partner was born in 1955 or earlier The second partner may have been born in 1960 or earlier
Senior V 75+ At least one partner was born in 1945 or earlier The second partner may have been born in 1950 or earlier
 

Junior MAC Age Categories

Pre-Teen I Born in or after 2011
Pre-Teen II Born in 2009-2010
Junior I Born in 2007-2008
Junior II Born in 2005-2006
Youth Born in 2002-2004
Under 21 Born in 2000-2004
 

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.