Calculate Age Based On BirthDate

function SetAge(birthdate) {

            var birthd = Xrm.Page.getAttribute("birthdate").getValue();

            if (birthd == null) {

                return;

            }

            var today = new Date().getFullYear();

            year1 = birthd.getFullYear();

            Xrm.Page.getAttribute("new_age").setValue(today - year1);

        }