[Dansong]

At what point (in years) does the quality actually start to decline?

[count-your-change]

About as soon as it goes into one of those plastic milk jugs so it’s best aged until the jug is about half full.

[antiRepublicrat]

At what point (in years) does the quality actually start to decline?

I don't know if it ever does. But after 70 years there can't be much left of what was in the cask. You get about 5% evaporation per year. So while there may not be a quality limit, there is a practical limit.

[Mariner]

"At what point (in years) does the quality actually start to decline?"

Whiskey in the bottle does not change unless exposed to UV light (sun). It gets worse under that one circumstance.

Whiskey in the barrel gets better with each passing year...mellower...richer.