Huck tells us (indirectly) how old he is in the narrative . . .

Adventures of Huckleberry Finn - Chapter 17

     "Buck looked about as old as me --- thirteen or fourteen or
along there, though he was a little bigger than me. He hadn't on
anything but a shirt, and he was very frowsy-headed. He come in
gaping and digging one fist into his eyes, and he was dragging a
gun along with the other one."

The kid in the Disney film is 11 years old.

KB