Answer 1

You can envelope the input part of your code in the while loop with a try/except phrase within, to ensure that the input is a number, ultimately.
With that done, you might get down to 'if' statements.

Answer 2

because when u enter a string, the first line tries to convert it to an int, so it crashes the program

Answer 3

try:
age = int(input("Please enter age:"))
except ValueError:
print("please enter a number")
if age >= 18:
print("access granted")
else:
print("denied")