class Person:
    def __init__(self, initialAge):
        self.age = initialAge
        if self.age < 0:
            print('Age is not valid, setting age to 0.')
            self.age = 0

    def yearPasses(self):
        self.age += 1

    def amIOld(self):
        if self.age < 13:
            print("You are young.")
        elif self.age >= 13 and self.age < 18:
            print("You are a teenager.")
        else:
            print("You are old.")