IF( MONTH(Today()) > MONTH(Birthday__c), YEAR(Today()) - YEAR(Birthday__c), If( AND(MONTH(TODAY()) = MONTH(Birthday__c), DAY(TODAY()) >= DAY(Birthday__c), YEAR(TODAY()) - YEAR(Birthday__c), YEAR(TODAY()) - YEAR(Birthday__c) - 1 ) )
FLOOR((TODAY() - Birthday__c) / 365.2425)