How multi factor authentication work for existing users

If I enable multi factor authentication, how old users will be able to login or setup their google authenticator app ?

Upon login they will be allowed to enroll using Google Authenticator and then after enrollment they will need to provide the second factor of authentication. In subsequent login they will only have to provide the second factor as they are already enrolled. If you haven’t done so already you may want to check: Troubleshoot Multi-Factor Authentication Issues