It's going to have to happen sooner or later. They can't be kids forever, unless they want to make it as unrealistic as possible (as if they haven't done that enough already).
I strongly disagree on both, the idea of the characters growing up as a mere matter of time as well as on the desirability of their growing up.
The characters lived through the time which it took for the Great Valley to regrow twice (once after it was devastated by fire), through the time it took for a newly hatched sharptooth to learn how to talk a foreign language, through at least one cold time (but judging from the other facts and Petrie's LBT 7 statement on five cold times being a very long time we can take it for granted that LBT 8 was not the first and only cold time they saw), and several seasons through which the treesweet tree regrew (at least three times). After they have lived through all this without showing any sign of growing up I don't think it is going to happen at any time as far as the movies are concerned.
While this may not be very realistic I must say that I feel much better about keeping the characters basically as they are (with the degree of learning character developement which we have seen in earlier movies). I see no need for them to develope any teenager "airs and graces".
There is yet another point to suggest that they are not going to grow up (though I must admit that I don't like that point). If they continue the process of making the movies for a more exclusively young audience they are not going to change the characters by making them older to include topics of no particular interest for the younger audience.
