I'm pretty sure that in H&HB the Pevensies are young adults. At that point, Lucy's around the same age as Aravis, who's probably a preteen.

As for the actors aging, I'm a bit concerned how they're going to make it work out, unless they choose new actors. In interviews, Skandar's voice is starting to change, and by the time the next film is churned out, Anna and William will be adults, in a sense. I mean, they're already 17 and 18, respectively.