When do I need to create Apple Developer Account? Say I want to start working on some app X which I want to submit to App Store.

Is it necessary to have a developer account, before I can start working on this app?
A free Apple Developer Program membership now allows you to develop apps and test them on your devices. A paid membership allows distribution.

A developer account is not required to start developing apps, however not having a developer account whilst developing the app can have the following downsides:


You will not be able to build the app on a real device (unless you jailbreak).
You will not have access to the latest developer builds of iOS and Xcode, or their documentation (a free account will only allow access to the latest public builds and their documentation).
You will not have full access to the Apple Developer Forums (a free account will allow this for certain sections).
You will not be able to create provisioning profiles for services such as PhoneGap Build.


As per above, certain features are accessible with a free Apple Developer Account. You can sign up at Apple Developer Registration.