To go back to Essex's point about the ages of the hobbits and also that the 'coming of age' was 33. At the time of Tolkien's writing, our coming of age was actually 21.
So how would we translate the ages of our hobbits into 'man years'?
By my reckoning, the actors playing Merry and Sam should have been, or at least look, in their early twenties, and Pippin in his late teens.
Obviously Frodo should not look like a fifty-year-old man, but he should look older than the other hobbits, in his early thirties perhaps.
In the film, I think Pippin looked about right to me, but Merry and Sam were a shade too old and Frodo much, much too young.
And to go back to the original question of the thread (of course!
) movie-Frodo did grow somewhat, but should certainly not have started out giving the impression of being a schoolboy. Which, I agree, he did.
Because (as I said before somewhere else) would Gandalf, Galadriel and Elrond really have entrusted the most dangerous thing in middle-earth to a schoolboy?