Many of the peat phenols (or is it esthers?) tend to break down over time in the cask, so peaty/smokey scotches actually can lose flavor over time — though with a good cask they can pick up woody complexity. Younger islas tend to be more vigorous!

Edit: I don’t believe there is a requirement in the law to only use previously used barrels. They are just cheaper (boubon IS by law required to use only new casks, which results in a lot of availability), and aren’t quite as active, allowing for more gentle maturation.