[Mithalwen]

If I dare mention the S word, one reason why Bilbo seems perpetually childlike because he is unmarried and in the context of Middle Earth, therefore more or less by definiton, not sexualised. However he doesn't have Frodo's ascetic quality. In one respect he is prematurely an old bachelor, too content with his own self indulgent routines to contemplate change; in another he is a really like a child in his innocent enthusiasm for adventure stories and firework parties.

Tolkien's explanation for his not marrying given via Gandalf is that he needed tobe free for adventure, however even when he has had his adventure, Tolkien rejects the solution of marrying him in order to produce an heir. Pehaps at that stage LOTR was still intended to be more of a true sequel to the Hobbit and in order that children could still relate to him, Bilbo had to be kept away from the true adult role.

However it does seem that in a sense, the Ring not only stopped Bilbo growing old - it stopped him growing up. If 50 is the hobbit 35 then the quest of Erebor is a genuine midlife crisis -which is never quite resolved. Bilbo goes more or less directly from the end of youth to death.