Briar oxidizes (as most woods do), changing color with time and exposure. Speaking generally, newly boiled pieces are whitey/pink, old pieces are inevitably a sort of mahogany color. I posted this picture before:
Clockwise, from top right: Greek, 30 yrs. Italian, 2 years. Italian (same mill) 6 months - notice the oxydized "skin" only as it dries. Bottom left is Italian with a few years on it from the mill that leaves maybe the whitest briar in general, and upward on the left is a year old block from Mimmo and a 10 year block from Mimmo (top left). Some of this is regional variation, some of it is the particular process, but basically it all drifts toward brown.
It gets harder as it does so, and it becomes more and more dimensionally stable (usually - I have had a few exceptions, pipes from the 80s that moved all over the place). Cutting new briar you get ribbons, cutting old briar you get chips.
Is there a vast difference in smoking? Yes and no. That old greek stuff tastes pretty earthy to me, I prefer the cleaner tasting Italian wood, even new. Do I think briar should sit for a year or two before being made into a pipe? Yes I do, I get more positive feedback on those pipes than on fresher stuff, for the most part. But you could never stick a pipe in your mouth, light it and say "Oh, this is 12.5 year aged briar."
And, for the record, ALL briar is "50 years old" in that the plant has to be a certain age before it's big enough to be worth harvesting. No one ever sat on and cured burls for 50 years. No one. Ever.