The mistake in this Python code is that the input function returns a string, not an integer. Therefore, when you compare the value entered by the user to an integer (2000 or 1999), it will result in a TypeError.
To fix this, you can convert the input to an integer using the int() function. Here's the corrected code:
Birth_Year = int(input("What is your birth year? "))
if Birth_Year <= 2000:
print("Wow, you're old.")
if Birth_Year >= 1999:
print("lol, you're still a baby.")
Note that I've also added indentation to the print statements so they are executed only when the condition is true.