How do I print a returned variable calculated in another function?

[j.crater]

yearAt100 is local to the function yearThatYouTurn100. Once the function is over (after "return"), the variable ceases to exist for the rest of the program.

So what you should do is store the returned result in a variable, which you can then print:

def main():
    result = yearThatYouTurn100()
    print(result)

result will hold the value returned by the function.
This principle works generally for functions, and for any data types you might want to return.