Hey by declaring the initialAge argument as an instance variable within the constructor, it can be used by the other class methods instead of using the global keyword. Check out my solution:

class Person:
    def __init__(self,initialAge):
        # Add some more code to run some checks on initialAge
        self.age = 0
        self.initialAge = initialAge
        if initialAge < 0:
            print ('Age is not valid, setting age to 0.')
        else: 
            initialAge = self.age


    def amIOld(self):
        # Do some computations in here and print out the correct statement to the console
        if self.initialAge < 13:
            print ('You are young.')
        elif 13 <= self.initialAge < 18:
            print ('You are a teenager.')
        else:
            print ('You are old.')


    def yearPasses(self):
        # Increment the age of the person in here      
        self.initialAge += 1