A barrel supposedly has a useful life of about fifty years. One that has held bourbon for only two years will have more "wood" still in it for the whisky, that is to say more influence in a shorter time. Scotch producers will fill a barrel three, four, possibly five times, depending on how long they are used each time. I saw at the Balvenie cooperage how barrels were scraped out and recharred after the third of fourth filling, to get one more use out of them. Later fillings will not be the best whisky, probably meant for blending--I'm quite sure I've had some IB whiskies from such tired casks, which should never have been used for a single malt.
To answer the question, I imagine they'll take whatever they can, and that the majority will have been used two or three years, since that's what the majority of bourbon produced is. Whether older barrels are more or less desirable is a good question. I would infer that they will have a shorter life in Scotland, and less and slower influence, and thus in theory would be less valuable. But I'm guessing, and don't really know what the difference in terms of bourbon influence would be. I think the bourbon itself is supposed to take the edge off the wood, but be fairly neutral in terms of its own influence. Too much may not be a good thing.