Returns the user's age, determined by their birthdate()

def age(self):
        """ Returns the user's age, determined by their birthdate()
        """
        if not self.birthdate():
            return -1
        adjuster = 0
        today = date.today()
        birthday = self.birthdate()
        if today.month == birthday.month:
            if today.day < birthday.day:
                adjuster -= 1
        elif today.month < birthday.month:
            adjuster -= 1
        return (today.year - birthday.year) + adjuster