Preserve or Grow?

When writing a series, I've come across a bit of a problem. My characters are both juveniles and some of my readers suggested that in order to keep the 'timelessness' of my books, my characters needed to remain as juveniles and technically never grow up. (Personally, that made me think of Peter Pan and how he never wanted to grow up). On the other hand, my editor said she looks forward to watching my two characters grow up and that leaves me in a bit of a dilemma. Do I keep my characters at sixteen and nine for the entire series? Or do they grow up as people do in reality? I mean, I don't want my books to lose whatever timeless feeling they may have. But on the flip side, I'd like to be a little more realistic when I write, even if it means them growing up. Have you ever been in a pickle like this before? How'd you handle it?