Stock is supposed to be rotated, so newer incoming products get put in the back, older products get picked first. Doesn't always happen that way. I watched a teardown of a pre-built, the bios on the motherboard was over 1.5yrs out of date. On an AMD pc, that's nuts, surprised the newer ram even worked.
The only guarantee you have about bios revision age is that it will be older than the actual manufacturing date of the mobo. Companies box the mobo and ship it as is, they will not update the bios, nor will whomever is selling it as new, unless it's included and specified in a pre-built or custom build and the builder did it for you.
So what bios you get depends on the stockers, rotation, volume of sales of the mobo, age of the platform, amount of revisions since the manufacturing date etc. Totally pot-luck on your end regardless of purchase date.
Because of that, 3rd parties like PCPartPicker can't say for certain what bios revision you'd get, whether it's compatible for certain cpus or ram etc. Not until a platform has aged enough until the chances of getting a really old revision are microscopically minute.