It all comes down to how someone's age reads onstage, and what the director envisions for that specific production. Just because Laura Benanti and Lauren Ambrose can convincingly play Eliza Doolittle at 40ish does not mean that any 40-year-old soprano could. Same with Bernadette Peters playing Annie Oakley at 51.
In Follies, Sally and Phyllis are written as 49, but the main 2 couples are often cast with people in their 50s-60s. Depending on the actors and the way it's directed, it usually works. Not a musical, but at age 86, Robert Morse played a role written for someone in his 30s-40s in The Front Page. It didn't really work, but it was fun seeing him onstage.
Hairspray0901 said: "Most shows with teenage characters areplayed by actors in their twenties and sometimes thirties. Take Hairspray, Mean Girls and Wicked for example. "
Dont forget the two female leads in Frozen, who should be 18 or 21 (at the oldest).