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)