I can’t really say which way is best but I do tend to like characters that mature more on-par with the rest of us when I am reading a series. It doesn’t have to be a year per book but it helps to know time goes on for the characters I love, too.
There are a few exceptions to my feelings, though. First, would have to be a series aimed at younger children. If you made the characters all grow up, it would have to become a very different type of book if it was a long-running series. A series with a formula that relies on things staying still is almost promised not to bend to the wishes of Father Time.
Do you guys want your favorite characters to stay forever young?