I think the most important element which will affect the taste of your whisky is when air comes into contact with it. If a cork is used, then over time it can deteriorate, allowing air to seep in and the cork itself may affect (flavor) the taste. If an old metal cap is used, then over time, that too may deteriorate or rust causing the scotch may take on a metallic taste. So as you can see, there are several influences which can affect it. Does sunlight weaken the spirit or change it? My understanding is that leaving it in the sun is just not considered an ideal way to store it. I myself have never tested this - and never plan to! I certainly don't know anyone who would choose to leave their scotch in sunlight for a long period of time...do you?! Many really old whiskies should be perfectly drinkable - just keep in mind what influences can possibly play a part in altering the taste.