Does anyone know of a math technique to determine if someone is 21 or over by their birth date on their ID

From taking a online class I am aware of one technique such as;
taking the given birth year and adding twenty then adding one to that result
Example if someone was born May 5,1982
you would do --> 1982 +20 = 2002
then-----------------> 2002+1= 2003
2003 to 2004

calculated year before current year =21 or oder
calculated after = underage


I know it may seem as though I understand but I can't seem to do this without pen and paper, and long thinking. Can someone please break this down to me I would really appreciate your help. Please no smart answers or I will report you.

Thank you to those who do help (:

If it says 1990 or below there good. If it says 1992 or higher there not. If its 1991 check the month. other that your good just remember 1990

Just take the year the person was born and subtract it from the current year