Girardin makes younger and older lambic available (as well as krieklambiek) in bags in box, but this is only available in Belgium as they've asked places like BiaB to stop shipping them abroad as they don't believe they travel very well, as others have noted already. Keep in mind that young lambic will ferment in the bag making it a bit explosive. Whether or not the webshops respect those wishes is another story, but yeah they mostly come in 5L bags. The system is actually pretty simple and it seems most brewer/blenders know where to get ahold of unused bags in box to fill on a whim. For example, Pierre took four different bags in box (three component lambics and his blend of them) to Amsterdam for Carnivale Brettanomyces in June. He filled them up just a couple days before from his barrels. Others that are or have been available have been Lindemans, De Troch, Timmermans, and Beersel but to varying degrees of availability. If you're local it's also easier to get them on demand. You'll also sometimes see locals coming and filling up plastic jugs with spigots on them. I once knew a guy who used to go to Lindemans and get waterbottles filled with lambic just to drink.
They definitely don't do this anymore except for some old locals who had been doing it for years and years. There was one older guy who came in every time I was there on the last trip getting his brown plastic jug that looked like a barrel filled up. Most of the time when you see Cantillon lambic referenced anymore on those kinds of blends it means they've taken a bunch of Bruocsella and put it into the blend.
I think it's a combination of knowing what's in your barrels, how they're behaving, and when/how you think you can blend them or blend out any of the off flavors that are in there. There are definitely older barrels though that get bottled individually or whatever (See:
LH12). You know the phrase they have written at Cantillon. Le temps ne respect pas blah blah blah. I know that Girardin has let some barrels go and never used them/got rid of them because they were three+ years old.
Tilquin gueuze is tpically comprised of 50% one-year old lambic, 30% two-year old lambic, and 20% three-year old lambic, so that gives you a rough idea. And even within that breakdown, Pierre generally uses a pretty specific ratio of barrels whose wort came from different places. I am pretty sure I have notes on the Cantillon ratio for the gueuze as well that I need to dig out. When Jean talks about blending gueuze he talks about blending it to an average age of lambic, but I forget what the exact number is. Interestingly, Jean went outside the norm a bit when he did the Chouke by using equal parts 1, 2, and 3 year old lambic.