Liquor doesn't age in the bottle the way wine does. Once it's in glass, it doesn't change any more. A five-year-old whiskey, left in a bottle for twenty years, is still a five-year-old whiskey. A five-year-old whiskey, left in the barrel for twenty years, is a twenty-five year old whiskey.

Other than cream liqueurs, if a liqueur is sealed tightly, it shouldn't age too badly. They get nasty if they oxidize or if they evaporate. Over time, that happens, but, hopefully, not enough to matter.