[Atariel]

I'm not quite sure where this came from- it certainly wasn't my brain (it being too complex and intelligent for me to think of)- but someone once said that those who read the Hobbit as children would have been adults by the time the Fellowship of the Ring was published in 1954. So, if the elves are all giggly and childish in the Hobbit, then all melancholy and serious in LotR, perhaps they aged with the story.
But, then again, most elves are thousands of years old, so a matter of seventeen (real life) or even eighty (M.E) years would seem like nothing. And, to reiterate everybody else's point, the Hobbit is ultimately a children's story, whereas LotR wouldn't necessarily be described as suitable for most children.

Thankyou to Turin Turambar for starting this chain, because it's very interesting and it's something on the subject of which I've always wanted to know other people's opinions.