Can I calculate if a person was 21 at the date of 1/1/08 or 7/1/08?

Also, can you tell me how to convert SSN numbers that were imported from a
database to a number format and no matter what I do by trying to format the
cells it does not change. I have to go in each cell and take out the hyphens
but I have over 4K rows.

I have tried right clicking and format cells, special paste and nothing.

SSN LAST NAME FIRST NAME BIRTHDATE
123-45-5678 SMITH JON 10/14/1977
123-45-5679 DOE JANE 02/21/1982
123-45-5680 DOE JON 01/31/1980
123-45-5681 SMITH JANE 07/27/1980
123-45-5682 SMITH SARAH 02/24/1980
123-45-5683 DOE ANDREW 01/24/1988
123-45-5684 DOE STEVEN 07/07/1985
123-45-5685 DOE SANDRA 07/08/1989