[lathspell]

Adolescence is only by law gained at a certain age. To act like an adult is ofcourse quite a different thing. As Laitoste pointed in a very good way:

There's a fine balance between growing up too fast and not growing up fast enough.

The 'weights' that make this balance either go slightly to 'too fast' or 'too slow' are the events that happen to you in life and, I believe, in a way also IQ matters. Yet the events seem to play the main part in all this. Two examples of this are a friend of mine and (not to be haughty or anything) myself.
1. My friend (and I think some here still know him as Savalas) was always energetic, allert, keen and in a way 'childish', yet since somewhat more than a year someone close to him is terminally ill, and since this happened he has grown an 'adult' extremely fast.
2. I, myself, have experienced too much in my life and at a young age (death's, seeing someone die, abuse (of which I won't tell further over a forum!)). This all, in a later stadium when you begin to notice the extravagancy of these things, making me older in person than I should be. Now I'm 19 and I'm the first Dutch person ever under the age of 21 to work as security in one of the Dutch ministries, because 'I appear older'. (Again: I say this for the topic. Please don't take me as an arrogant 19-year-old.)
Another important point is when these events happen in life, which (if for reaching adolescence) should be in the Time when you are aware that you are growing up. That would be from about first schoolyear (age 4) until you have reached adolescence by life-experience (older age).
The point I'm trying to make here is that it matters how you live your life and what experiences and events happened in it when determining someones personality on aspects of 'adulty'.

Children and childish manners seem to have played a great part in Tolkien's books (and maybe also in Tolkien's life). There are many examples possible in which Tolkien chose children to do deliberate things, and a couple are already mentioned in posts before me, like: The Cottage of Lost Play from the Lost Tales... The Twenty-four Feast from Smith of Wootton Major... and the Garden of the Dark Side of the Moon in Roverandom. Why this is so I can not guess, for I haven't lived in close contact to Tolkien of one of his sons, but children certainly seem to have some part in his stories and they are small but, if interested, important.
The longer childhood of Hobbits are one of these examples as well, but maybe this one is explainable. The Hobbit-society was very different to the one we live in. We, and already in Tolkien's time, live in times where everyone needs to produce and consume and you know there's always a war going on somewhere. In opposite to this the Hobbits live in a World where they have time to do things as they wish and to arrange things and meet people at times set by them. And also very convenient is that they were for the greater part unaware what happened beyond their borders, most of them never even went out of the Shire. Therefore they were only bothered with the goings on in their own little country, which was almost always peaceful and jolly. This conludes in the fact that they experienced little of the events I mentioned above (not meaning they didn't happen ofcourse) and thus had a longer average childhood.
Imladris makes a point I didn't think of myself but want to back-up immediately:

he grew up during two world wars or so (right?) and children would grow up fast in those kind of times.

Tolkien might have 'created' the Shire in a way of showing his feelings about the Wars, especially the WWI, because the Hobbit was written or finished in that time. Maybe he used the Shire as an outlet of his feelings.

*** I could not find a date of time when Gollum's people settled near the Gladden Fields. If someone knows this, please tell, for it will determine if the next piece of text can possibly be true. As I did not find it I took it for a possibility ***

The smallness and childishness of Hobbits can also be explained evolutionary. Since they were of old a people that lives near the Gladden Fields as Gandalf says in FotR - the Shadow of the Past, they would 'inherit' the appearances and features of those people (of which Gollum was one).
These people live near the Gladden Fields, which is a very large swamplike area. This would mean that, quite likely, the greater part of their food supply would be 'fish' (also in accordance of Gollum's liking for it even before going after Frodo). A fanatic fisherman knows that fish get scared away if you make too much racket or if you lean over the riverbank. The 'Ancestors' would be depending on their fishing capabilities and become stealthy and masters of silency. Nature, however, has developed 'evolution'. This would mean that over a very long time the 'Ancestors' would be growing to the appearances and features they needed there, meaning they would become naturals in making no noise at all and becoming shorter.
Fish is considered healthy by many people and scientists. This, and the fact that they very likely had no natural enemies (being at the top of the foodchain just as humans) would mean that they required longer lifes, because of their healthy food and environment. When passed over the Mountains and settled in the Shire they had no enemies whatsoever (except in the Fell Winter and the Battle of Greenfields) and were almost completely shut out from the World Outside, giving them (as said above) less important experiences making someone an adult and thus gaining a longer childhood. And probably that because of this older Hobbits remembered how to be young for a long time as well.

Gandalf made his choice of 'burglar' very delibirate, because Bilbo was always 'an adventurous lad'. Hobbit standards got a hold of him though and he became 'an adult' thinking about his prestige towards his neighbours and such things. After his journey with Gandalf and the Dwarves (which is an event that changes life and at younger age makes you an adult) made him look at life differently, changing his priorities and he became 'weird', according to the other Hobbits except possibly the Tooks. The extraordinary thing is that when Bilbo became 'weird' he was again as if he was young, only a good deal wiser.
But here again I do not know why it is that Tolkien especially chose childish manners.

And: Mark12_30... great topic!

greetings to all,
lathspell