Use "whenever" to talk about something that happens again and again in similar situations. For example:

He likes to stop by our office whenever he's in New York.

Individuals should be supported to make their own decisions whenever possible.

You could use "when" in each of these situations, but when you use "whenever" it sounds like it happens almost every time. If you use "when", it sounds like something might happen in that situation, or might not. So, for example:

I get a sharp pain in my shoulder when I raise my arm.

If you say this, the doctor might ask, "Does it hurt every time?" But if you used "whenever", the doctor would already know that it was every time.