code: function getAge(birthday, when) {
    birthday = this.toDate(birthday);
    when = this.toDate(when || new Date());
    var b = new Date(birthday).setFullYear(2000);
    var w = new Date(when).setFullYear(2000);
    return when.getFullYear() - birthday.getFullYear() - (w >= b ? 0: 1);
  }
docs: 年齢を返します
  @method getAge
  @param  {DATE} birthday
  @param  {DATE} when
  @return {Number}      age