The answer is: "it depends". If you birthdate variable is a character-type, you will need to convert it to a SAS numeric DATE variable using the INPUT function and the appropriate SAS DATE INFORMAT. However, if a SAS PROC CONTENTS on your SAS database file indicates that this variable is SAS NUMERIC and is using some SAS format to display (externally) the formatted value, then you can use SAS functions, such as INTCK, to determine age.
Have a look at the SAS support website
http://support.sas.com/ and the SAS Base documentation in the SAS Language guide. Also, at the support site (using the SEARCH option), there are technical papers (SUGI, SGF - user group conference proceedings) which explore this topic in great detail.
Scott Barry
SBBWorks,Inc.