What's so special about '50'?

[Guinevere]

I quite agree with Child of the 7th age and Phantom(and the others)
Bilbo, before departing on his 111th birthday, says about Frodo:

"He would come with me (...) in fact he offered to (...) but he does not really want to yet.(...) But he is still in love with the Shire, with woods and fields and little rivers. (...) I hope he will be happy, when he gets used to being on his own. It's time he was his own master now."

Clearly, Bilbo thinks that at that point Frodo is not mature enough yet for leaving the Shire, but as he uses the word "still" he expects obviously that after a time Frodo will not be content anymore in the Shire and will have the urge to go on an adventurous trip.

At first I felt this was a bit strange - one would expect younger people to be more adventurous, and middleaged peole more settled and content.
But perhaps this is only so because young people have usually more freedom to travel about and make experiments , and when people are older they have a job and a family and responsibilities so they can't just take off so easily anymore, even if they'd like to!
That's probably why Tolkien made both Bilbo and Frodo bachelors without a family to tie them.
And Frodo finally does become a bit bored in the Shire:

"I should like to save the Shire, if I could - though there have been times when I thought the inhabitants too stupid and dull for words, and have felt that an earthquake or an invasion of dragons might be good for them."

he tells Gandalf.
Well, if he felt like that about them already before the quest, no wonder he couldn't go on staying in their company afterwards...

I also agree very much with Liriodendron's posts.

For me personally, this "taking off at 50" is a funny coincidence, because I read the Hobbit and LotR for the first time at the age of 50!! Discovering Tolkien was quite a revelation ! It was just what I needed at this point of my life - it opened up a whole new world and made my life so much richer . Not only did I set out to explore Middle-earth and learn more and more about Tolkien, but thanks to this I also discovered the internet (and therein the Barrowdowns) (Before becoming engrossed with Tolkien I had never even used a computer!) Once I travel THERE it is often quite hard to com BACK AGAIN !