I gotta agree with you on both points...
I got the impression from the books that he'd be around 30 as well, and dejah would be early 20s...
I think the rationale is that they since they were planning to do the entire franchise, you'd better start with a young actor so he can more or less hit the average target age throughout the entire run as JC isn't supposed to age (he's virtually timeless in the books).
They apparently didn't care much about that rationale with Collins though. Holy cow... she's gonna be positively ancient at the end of the series. Martians supposedly also have very long lifespans, so she wasn't supposed to age much either... who knows that they were thinking with her... the only thing I can guess is they wanted someone that could pull off the whole "regal" thing with the acting chops to give it some authority. You don't want some ditzy blond in the role that looks nice, but is a hollow actress, so they probably went with an older one for the experience.
Or they just got a great two-for-one deal by hitting up the X-Men Origins: Wolverine movie people... sort of a "Well, you can have Hugh Jackman for $$$, or for the same price, you can have both Kitsch and Collins! Heh heh... that's a joke of course...

-Mike