YEAR(CURDATE())-YEAR(birthday)-(RIGHT(CURDATE(),5)<RIGHT(birthday,5))

year( from_days( datediff( now( ), birthdate)))
 
now() 当前时间,精确到秒
 
datediff(begindate,enddate):返回两日期之间相关的日数.
 
from_days(days):从0000-00-00开始,根据传入的日数,返回一个日期;
 
year(date):获取日期上的年.