It would be nice if men and women were cast with no "real age" in mind. But the truth of the matter is, the movies are an age-ist industry. My guess would be that the studio execs are calculating/have calculated what the opinion of the age group they are primarily aiming at might be. And wouldn't that be younger than most of the people here at this board?
As you've noted, there seems to be no economic backlash coming from filling a major role in Tolkien's stories with someone significantly younger than the book dictated. In this situation all they have/had to do is question which would gain them more money: a younger actor or the name of Ian Holmes.
It sounds like they may have decided that younger won't hurt the profit margin as much. Also, does anyone know if unofficial feelers were put out, and perhaps Ian Holmes wasn't interested in the part? I would guess yes, but it is possible he was comfortable with playing a smaller part, as a mostly-older Bilbo, but not interested in starring in a Hobbit film.
I have no idea. But I do think of the reediting of Star Wars, and the replacement of the actor who played Darth Vader with the image of the younger actor who played him in the prequel. Again, I'd tend to suspect people like me were the rarity: those who would actively dislike such a replacement to the point of not buying the DVDs in question.
In short, the sad truth is they may not care about continuity nearly as much as pleasing a younger audience.
But all of that is just my guess.
