[Python] How do I compare a string and an integer? [duplicate]

I am quite a newbie in Python. I wrote this and got this error when i typed a letter in the input:

TypeError: unorderable types: str() >= int()


Here is the code that I wrote:

user_input = input('How old are you?: ')
if user_input >= 18:
print('You are an adult')
elif user_input < 18:
print('You are quite young')
elif user_input == str():
print ('That is not a number')

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