Sorting by age issue.

I have a table that stores users birthdays, date, month year, etc.

I then have a function which turns these into an age, I.E. 24.

Problem is when I want to show all users who are a specific age I get stuck. I can do it an easy way and sort by their year born, but this will sometimes show a fluctuation because everyone born from 1988 may not be 20 yet.

Is there any work around for this?