Yes, he's immortal. Most characters in Pokemon are, actually. This is especially true for the series' main characters. As in most video games and shows, the main characters don't [can't] die. Sure, Ash is technically killed in the first Pokemon movie by Mewtwo, or so it is suggested, but then what happens? He is revived moments later. Also, Ash's Butterfree dies, but that's a story for another time. One should take note that Pokemon's target demographic is children. Death + children's show = ☠ Not good.
Ash is forever young, because that's how most anime/cartoons work, or at least they use to. The main character remains the same age throughout entirety of the show's run with relation to the target demographic. I guess it is assumed from the creators' perspective that kids are less likely to enjoy Pokemon if their favorite main character grows up to be a hairy 22(age?) year old man. Such a move may be too large of a risk to take for the series.