Is it possible to turn off the Two-Factor Authentication for an Apple ID?
You are no longer able to turn off two-factor authentication beyond two weeks from enabling the feature. Previously, this was limited to accounts created in iOS 10.3 or macOS Sierra 10.12.4 and later.
Please refer to the Apple Support Document, Two-factor authentication for Apple ID for technical details regarding Two-Factor Authentication.

Can I turn off two-factor authentication after I’ve turned it on?
If you already use two-factor authentication, you can no longer turn it off. Certain features in the latest versions of iOS and macOS require this extra level of security, which is designed to protect your information. If you recently updated your account, however, you can unenroll for a period of two weeks. Just open your enrollment confirmation email and click the link to return to your previous security settings. Keep in mind, this makes your account less secure and means that you can't use features that require a higher level of security.