Javascript Get Age for over 18 or over 21 DOB validation

getAge.js

/*
*  Pass a date string into the function and return the age
*  dateString = MM/DD/YYYY
---------------------
*/

function getAge(dateString) {
  var today = new Date(),
      birthDate = new Date(dateString),
      age = today.getFullYear() - birthDate.getFullYear(),
      m = today.getMonth() - birthDate.getMonth();
  if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {
      age--;
  }
  return age;
}