Pete's right about Frodo's aging more slowly...but even Merry and Pippin should be older, and certainly Samwise. They all look like teenagers in the movie, which corresponds to the twenties for Hobbits (tweens). But in reality, they were that age at Bilbo's party, not seventeen years later. What the movie did was to eliminate the seventeen years between Frodo's getting the ring and setting out on the quest. There is no suggestion at all that such a long time has passed. So Merry and Pippin and Sam are all still kids. I think they did this partly so they could cast with twentysomethings to appeal to that audience. Also, remember, if Bilbo was fifty when he stopped aging, and Frodo 33, then Bilbo should look roughly 17 years older. But they've picked actors who are about 60 and 20 for these roles, accentuating Frodo's youth once again.

As for Aragorn, as you say, it is often suggested how grizzled he looks. In fact, everyone who doesn't know what the rangers really are, including Butterburr, thinks Aragorn's essentially a bum..."Strider" is old Barleyman's word for "tramp," and Aragorn expresses anger at Elrond's council that such a soft fat man should call him such a disrespectful name. In the movie you never get the sense of Aragorn as a grizzled old fart, which is the way many others perceive him in the books. Again, the target audience respects athletic-looking, young-looking men, and that is what they're given. It's not a big deal, and it's a bit picky as Pete suggests to make a fuss, but I definitely think it's a case of going after that young crowd.