If you open it at 50 you will waste it, you'll have much better loot soon.
40 is around the right time as you'll be quite the way through playthrough 2 and as i found out, you'll need some upleveled items to complete some quests as you'll be dying a lot.
So yeah, use it around 38-44 but not before you start playthrough 2.
Noted and thanks for the tip.
I accidentally wasted my key on that chest as soon as I reached Sanctuary. I was just going on a loot binge and saw this here fancy-looking chest without realizing it's importance...
:'(
Anyone have a screenshot of the chest? As this is a mistake ive been worried about repeating myself.