I think everyone else already covered it but I think part of the reason that you don't see a lot of vendors selling "steeped" (aged, fermented, geriatric, over the hill, *ahem*) juice is simply because "steeping" (See above if you prefer different term) is a personal choice. One person may feel like it only needs a week, some a month . . .
I find the whole practice a bit un-necessary, however, I also have multiple bottles on hand at a time and don't go through a lot of them. So, I guess I "steep", i'm just not trying to, lol.
A lot of vendors are pretty upfront about "steeping" recommendations and that the flavors can change over time.
I've heard the shelf life for a bottle of juice stored properly is a year. Personally, if I ever have a bottle that lasts a year, I'm just tossing it because I obviously don't like it to not have gotten desperate enough to vape it when I ran low on other things, lol.