The Chipmunks went all in on Sludge, changing their band name to Chipmunks on 16-Speed to differentiate their new darker material from their old kid friendly stuff. Alvin is now Alice after transitioning from male to female and is also a user of weed, shrooms, LSD, and allegedly has done literally all the drugs, Theodore is a non-binary pothead, and Simon is seemingly straightedge and well-adjusted, but he's super chipcestuous for Alice and is still her self-appointed wrangler who simply makes sure she does Alice things safely. Alice's outrageous antics on stage and in the public eye, as well as her period of semi-reclusiveness before transitioning, has made her the subject of controversy, bizarre stories, and conspiracy theories. She often plays into them because she finds it funny and/or cool.
The Chipettes got normal jobs but still sometimes collaborate with the Seville siblings. Brittany is a fitness instructor, Eleanor is a chef, and Jeanette is a librarian and a member of the Communist Party of America.
They're in their 30s despite how long they've actually been around. This is due to Clyde Crashcup fucking up the timeline in that time travel episode, causing them to exist in the 60s and also being stuck as kids from 1980-2000 until the Y2K bug cancelled it out. Clyde didn't understand millenniums let alone his own invention.