Now that is an interesting point. Frodo was fifty, and of age. Wood was barely out of childhood! (though he went about lobbying for the role in a very grownup and determined way) PJ ignored some of the class distinctions that JRRT used as a device.
Merry and Pippin were in their unreliable "tweens" and were the kids of the book.
However, I think it worked out great, this liberty that PJ took. He wanted a "luminous" Frodo, and Wood had that transcendency. He was described by JRRT as "fairer" more "elf-like" than the average hobbit. There hasn't been a more beautiful and compelling set of eyes since Audrey Hepburn.
I agree that all the actors gave excellent performances with the roles they were given. However, many of the movie characters were significantly different in personality and motivation from the characters Tolkien created - some so much so that they were different people with the same names.
How much one is bothered by the alterations of plot and character made by Peter Jackson is, of course, a personal matter. It bothered me a lot!
I saw Frodo and Sam as about the same age in the film... and saw Frodo as being more sophisticated, undoudedtly assisted by my mind's eye background of what they should be from the books. I loved Elijah Wood's portrayal of Frodo. He can communicate more with his eyes than some actors can with great dialogue.