Your own functions can raise exceptions with raise. This is the right thing to do when a function is given values it can't sensibly handle. Failing loudly with a clear message beats quietly returning a wrong answer:

def set_age(age):
    if age < 0:
        raise ValueError("age can't be negative")
    return age

print(set_age(30))
print(set_age(-4))

Choose the exception that describes the problem: ValueError for a bad value, TypeError for the wrong type. Whoever calls the function can then catch it, if they know what to do about it:

def set_age(age):
    if age < 0:
        raise ValueError("age can't be negative")
    return age

try:
    set_age(-4)
except ValueError as e:
    print("Couldn't set age:", e)