Calculate age in a function

I am trying to calculate and return the age of a person. My code is:

    <cfset variable.age = '(dateDiff("yyyy",query.birthdate, now()))'>

I get a "If the expression is a string, ensure that it is within quotes. Error: report.ModStat(query.soc_sec)

is not a valid ColdFusion expression." error message when I try to run the report.

Any help would be appreciated.

Thanks.