this seems like a really interesting idea. i reckon maybe have some 'rare drops' that you can only get if you kill a monster in its very last like 5% of its lifetime... thatd give the 'gotta collect em all' players something to really strategise for or hope to get lucky with.
My opinion on enemy strength is that they should gain damage up till probably 60% of life cap, and become more and more tactical and harder to out-skill up till about 85% of their life peak, while degrading in power for the last 30% of their life.
So enemies would be hardest by far to kill at around 60-75% of their lifetime, but their drops would also have the highest grade at that point (better leather or meat for wolves or silj for spiders), unless it is say snake venom if they introduce snakes, coz the younger a snake is, the more potent their venon is (not sure if spiders r the same).
Then of course on the other hand, the older mobs would have aged hides and stuff, which could have a different use... maybe to craft lower tier gear or gear specialised for aomething different to the higher grade leather?