>>50341711Mallow, Kiawe and Lana were adapted in the anime at Ash's age, in Japan students are grouped according to their age. This was done to make the group dynamic more believable, like a real classroom.
Personally I like it because it reflects how each individual develops at their own pace, who hasn't had a friend at school who was almost as big as his/her parents or on the contrary who seemed to be years younger?
In this case Mallow is the early Bloomer and candy eye of the class while Lana is the weird girl who will become a beauty until college.