To calculate a person's age from their date of birth, you need to subtract their birth year from the current year and then determine if their birthday has past or not. The way I would do this would be to do the year calculation first and then add that number of years to the persons date of birth. Then I would compare that new date against the current date to determine if their birthday has past yet (if new date is less than or equal to now then they have had their birthday). If they have not had their birthday, I would subtract one from our year calculation.
There are a bunch of other ideas on Stack Overflow that might give you other options:
http://stackoverflow.com/questions/9/how-do-i-calculate-someones-age-in-c[
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That should get you going. Now you just need to pull the data from the database, do this comparison, and then display the results on your ASP.NET page.