The calculation of age from survey data may be as following:
AGE MONTHS=(Data of survey-Date of birth)/30.42)
where 30.42 is the average month length all over the year.
AGEYEARS=((Date of survey or interview-Date of birth)/365.25)
You should have the two dates (date of survey and date of birth) in NUMERIC format.
You also can use the option of integer in the beginning of the formula if you want the results to be in integer months or years (no fractions).