A couple of years ago I bought 2 tins off ebay that were 19 years old. I haven't actually tried the new as they don't sell it in the UK anymore but a tobacco buddy (thanks Paulie) sent me a 50g pouch to try. It's currently in a jar waiting to be smoked but I haven't had a bowl yet as I like to give it 6 months in the jar first.
Here's a side by side picture of the two. Aged is on the bottom. Even with my arse backwards photography skills you can still see the aged has gone very black.
I’m currently conducting an experiment in this. Check back in 5-10 years and I’ll have an answer. I very much enjoy it fresh though as @smudgersmissingleg says without a “tobacco buddy” I wouldn’t know. What a shame that would be.