Tom Finn, a 57-year-old baby boomer, celebrated his 50th birthday with a raucous trip to Las Vegas: He and his friends went ziplining, nightclubbing and played blackjack at swim-up tables in a pool at Caesars Palace. “I was a frat boy wanting to relive my college youth,” he says.

But when his wife, Debbie Finn, a 50-year-old Generation Xer, hit the milestone birthday, she marked it by hiking four to six hours a day in Portugal and Spain. “I knew I wanted to be hiking and to be outside,” she says. “I’m much more aware of...