More on brisket

Some briskets will not fold over no matter how much you age them, but usually with age they will become more flexible as the proteins break down. Temperature will have something to do with how they bend as well.

If you use the point, make sure you check that too as sometimes even if the flat is nice and bendy, the point can be as hard as a rock. On the other hand if the point is chuck full of fat, it will be hard as well, so you have to look at it as well.

If the brisky has all the other characteristics you are looking for and is stiff, I would buy it and age it. But you should always know the kill date of your brisket if you are going to age.

When I shop for briskets, I never take flex into consideration. At finish point of the brisket everything will be broken down and you will get a moist tender brisket no matter the stiffness. I have cooked a lot of briskets of all grades and ages, and our finished product is always the same if we use the same process.
What I look for is even thickness of the flat and very thick point.

My packer IBP does not go by kill date. They use pack date. Which can be 2-3 days after slaughter. But it will be on the case and not on each brisket.

I've also have had prime's that won't bend, but turn out awesome.