I was in my 20s when I decided I was gonna live life my way and bugger what others thought .

Nothing else matter at the same time everything matters I'm pretty sure our whole existence is made up of matter

When you decide to live for yourself

When you've hit bottom and lost everything and all your support group.

It's not the question of stage.
Just when the realisation dawns upon of each entity being nothing more than a bubble, just Nothing remains significant.

Metallica?

At death

everyone is different but I have yet to feel like nothing matters. & given my life now, nor can I see that I'm likely to feel that way. even if things were to go ass up because then I would have something else to focus on.

never

The stage in which you stop giving so many fucks.
( age flexible)