I’m an old guy, but I love loads of musicians younger than 40.
As long as music meets the criteria I love, almost nothing else matters to me.
That criteria being (no particular order): very high levels of musicianship, musical complexity, deep and broad ranges of emotional content conveyed.
Here’s a few.
The Contortionist - Very highly complex technical-metal, but certainly not lacking emotion. All 5 members are under 40.
Corima - A fantastic "Zeuhl" style prog band with, sax, violin, keys, drums, bass. Most members are under 40.
Mohini Dey - one of the best jazz fusion bass players on the planet at only 29 YO. There are videos of her on YT playing live at the age of about 11, and she could still outplay musicians 3 times her age.
Darcy James Argue - extremely creative jazz composer, and leader of progressive big band, Secret Society.
Esperanza Spalding - Great jazz bass player and vocalist.
Samuel Andreyev - Canadian composer of the avant-garde. Just barely over 40.
Elis Hallik - Another great avant-garde composer.
Matthias Kranebitter - More great avant-garde classical.
Nubya Garcia - 34 YO sax player operating in the "spiritual jazz" realm.
There is a collective of young jazz musicians here in LA called "The West Coast Get down", all members are right around 40. The most famous are Thundercat and Kemasi Washington, but they are all very talented.
Anthony Cheung - just barely older than 40. Avant-garde classical composer,
Tesseract - another very complex technical-metal band. Although they are complex, they don’t loose track of melody and emotion, Incredible musicianship.
The German heavy jazz-fusion band, Panzerballett, have some members under 40. These guys have serious chops.