Kat Braz, a graphic designer from West Lafayette, Ind., was visiting the Iolani Palace in Honolulu this past December when she said a guide scolded her for straying from the group. The docent, an older man in a Hawaiian shirt, insinuated that certain people on the tour were fat, she said, and mocked them for not knowing obscure historical trivia. Ms. Braz, 35, couldn’t wait for the experience to end. “I’ve never had anybody be so rude before,” she said.

More arts-loving baby boomers—educated, experienced and recently retired—are...