In music criticism "mature" usually refers to songwriters or their intended audience more than the music itself. Given how intensely loaded people's ideas about correct adult behavior are that's probably for the best, but it leaves me with a few songs I can't talk about in the words which immediately come to mind.
The Ropers deal in a kind of standing-wave pop music that probably owes most of its existence to Unrest (though there are precedents farther back, including the riotously ill-named Savage Republic). This song, with a sense of forward movement that's unusual for them, uses held cello notes and jangly guitar bits to create a weird sense of vertigo, as though, for all its exuberance and romantic optimism, this is how all conventional rock music sounds to people who would rather be standing still.
[From 1995's All The Time.]