Hmm, coming of age, you say? Interesting indeed!
Hobbits had very little desire to seek out adventure, so I don't agree with the notion that fifty years of age equal to the prime time for adventuring. However, Bilbo did mentioned that at the age of 33, Frodo was old enough to come to his 'inheritance'. Now, Frodo was an orphan, and Bilbo, being a bachelor, had likely as not adopted him as a foster son. But coming into 'inheritance' means many things other than just getting a few heirlooms. Bringing it in today's context, it meant taking care of an entire family business while daddy retire. 18? More like 25!
At the age of eleventy-one, Bilbo would probably appear like an old fossil in the eyes of the youngsters. This did not happen, because the Ring pickled him in olive oil...
er, I mean, made him look like he had never aged one day. But Bilbo could not have deceived himself into thinking that he was still 'young'? Hey, his adventuring spirit is still burning in him as he rushed off under starlight to visit Rivendell again!
I don't suppose age had much to do with hobbits 'getting queer' and rushing off into adventure... Gandalf said "You can learn all that there is to know about their ways in a month, and yet after a hundred years they can still surprise you at a pinch." Well, Prof T himself is pretty much like a hobbit; his 'wimsies' can shock us into a hundred debates after the original work!