In the book, there is a strong development of a father-son relationship, but that was distorted, even reversed, in the films because of the director's decision to depict Frodo as a youth. (Don't get me started.)
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 thought it was more of an older brother/younger brother, but you are right, they did reverse the ages in the film. In the book, Sam is younger than Frodo (who is about 50 in hobbit-years, which would be ~ thirtysomething in human years)