What's so special about '50'?

[the phantom]

Tolkien could just as easily have made Frodo a contemporary of his younger companions, but he didn’t. Why?....Perhaps it’s simply that making Frodo younger changes the focus of the story to a coming-of-age motif – and Tolkien wasn’t setting out to tell a coming-of-age tale.

That's kind of what I thought. Tolkien wanted to tell a different kind of tale, one in which he desired a character who was already of age and had essentially become who they were going to become. In other words, a grown up who had lost most of their mischievousness and irresponsibility. The mission Frodo had to complete wouldn't have been finished had Frodo still been a mushroom-pinching youngster (and the same thing applies to Bilbo). I don't think his mind would've been strong enough for the task due to limited wisdom and experience and having less practice at mastering himself.

And once Tolkien decided that he wanted his hobbits to be sort of middle aged- why not pick 50? It's a nice, big, round number (a number that I would be if you doubled my age and then added a few more years) .