Some of the music industry’s biggest players are betting that a new sound is ready to catch on. It’s called classic rock. The genre is being reinvented by young musicians, some of whom are barely out of high school, who are channeling bands their mothers and fathers grew up with. Greta Van Fleet is a rarity in today’s music business: An old-fashioned rock band that could, some music executives say, break into the pop world. The Frankenmuth, Mich., group is made up of 21-year-old twins Jake Kiszka, the band’s guitarist, and Josh Kiszka, its singer; their brother and bassist Sam...