What determines the level of maturity of a game is the personnel: pure and simple. Rules oriented to more mature groups only work when the group in question actually is mature.
I don't especially recommend D&D rules for mature RP dynamics; they're fine but other, lighter rules are often better for more mature and complex plots. The more rules you have, the more games are forced into hack and slash D&D tropes; if you want to move away from that, use rules that encourage different outcomes, reward different behaviours and, most importantly, do not circumscribe the GM's capacity to tell interesting stories. It's a constant struggle for me, in one of my games, to maintain a coherent symbol system, religion and natural philosophy when I am faced with hundreds of little rules eating into that. The D&D corpus, as it stands, works actively against the creation of a thematically consistent world.
My general view is: if you want a more mature game, change the people you are gaming with and everything will flow from there.