I'd change the query to something like
Code:
SELECT ROUND(AVG(YEAR(NOW()) - YEAR(birthday_search) - (DAYOFYEAR(birthday_search) - DAYOFYEAR(NOW())) / 365), 2) AS avgage WHERE YEAR(birthday_search) > 1950
replace 1950 with any cutoff date... on my forum where I was an old phpbb forum I have some birthdays in there of 1900 for some reason so that addition makes the query more accurate.