Amusingly enough, the “gimmick” of Merrily We Roll Along is that the story is told in reverse chronological order. So it starts when he’s a successful middle-aged man, but with a life of regrets and ends when he’s a college student full of hope for the future.
Granted, it doesn’t end with him literally as a child, but it’s funny to me that you’re saying “let him fucking grow up already” in an article about a show where all of the characters essentially age in reverse.
Edit: Also, the original Broadway cast were almost all teenagers. I think the youngest was 17 and the oldest was 21.