Roger is the one who ages most noticeably and turns slightly "moody" by the time they get to Great Northern. I don't think the time line is particularly considered. We get continuity, but not necessarily of time.

In terms of character development, again, I don't think there is that much. Dick & Dot go from being fairly geeky townie kids to being more at home "in the wild" after meeting the various other groups and being allowed more independence. Although I think most of the adults would probably have freaked if they'd known they were living in the old bothy in Picts & Martyrs.