As Zacsmaster states, whisky doesn't mature in the bottle. (So, if it was a 5 year old whisky when it was bottled, it's still counts as a 5 year old whisky now). However, it doesn't go off either, so please don't use it to clean the sink!
Because whisky doesn't mature in the bottle, there's no market for old bottles of whisky (unless the bottles themselves were unusual or 'limited edition'). Consequently, there aren't any books or websites which specialise in dating old whisky.
Anyway, Bells hardly ever change their packaging. The only clues you're likely to get from the bottle are from things like looking to see whether the alcohol content is shown as 'proof' or 'ABV'. If you Google long enough, or find a forum discussing whisky, you should be able to find out when the labelling was changed.
I can't really see much point in trying to work out how old the bottle is. Just open it and enjoy the contents ;-) Then follow Zacsmaster's advice and buy a bottle of the superbly peaty Laphroiag!
Chris