Best single-malt Scotch...

"improvement" is a value judgment. lengthy aging changes scotch, which can become very dark and can have unusual, intense flavors, often unlike what we normally think of as Scotch. you should try them for yourself. If you are ever in Edinburgh, go by the Vaults at Leith and try some Society undiluted, non-chill filtered, single malt, single-cask whiskys. Also, the Society has touring tastings in major cities in the U.S.
I consider Michael Jackson's guidebook more useful.

I have both Michael Jackson and Jim Murray's books. I have heard excellent things abou the Society in Edinburgh and there is also a Whisky Society spot in London.

Two things, good whisky in a good cask will usually age well for a long time while mediocre whisky in a mediocre cask will have mediocre to terrible results.

Either way, at some point in time the whisky will leach out too many tannins and other nasties from the oak and can ruin even an excellent whisky. Still 40-50-60 year old and older whiskys exist that remain more than drinkable but I believe these are exceptions rather than the rule. Most would be over the hill at some point before those kind of ages.

One of the skills of the distillery is to decide when the whisky is "ready".

Somewhere I read an article whose argument contended that cask management contributes 70% to the final quality of a whisky. I don't know enough on the subject to argue or agree but it is obviously critical to the end product.

Good to know Rja. Thanks.

If you are going to Scotland, the best distillery tour, not necessarily best whisky, is at Aberlour.

A word of warning to drivers, it ends with 6 small whisky samples, including 2 cask strength. I went 2 years ago and had to walk about breathing in lungfuls of fresh air for 2 hours, before I felt it was safe to drive.