Well adult in the more graphic violence type... which can be given a more subtextual approach.
http://lotr.wikia.com/wiki/Fourth_Age that's about all the info given by Tolkien himself that I know of.
Gives enough elbow room to move to dwarves into a more industrial part to fill. Hobbits I'm leaning towards them being looked down upon a bit as a race, becoming urbanized they collectively found their aptitude of stealth going to more of a rougish sub cultures.
Elves are a bit trickier though. I'd like only a limited number of players being an elf, mostly because them returning from the undying lands would be a big freaking deal.
I'd even allow an orc to join with some guidance on backstory. If you've played/beaten Shadow of mordor
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Then you know that some orcs will were shifted away from Suarons sway with that whole "white master" bit. Call me an optimist but I'd like to believe the ones that were "marked" then left to their own volition went awol after the war of the rings.