As a dozen other people will probably also inform Ed Rash, Huck does
tell us his approximate age:

"Buck looked about as old as me--thirteen or fourteen or along there,
though he was a little bigger than me."
--Chapter XVII (start of the Grangerford episode)

But Twain does some cheating here.  Huck is awfully savvy for his age
(maybe for ANY age!) in many ways, yet in other ways very naive.  Most
obviously--as countless critics have noted--Twain plays to genteel
values by keeping him nearly pre-sexual.

He's much fuzzier about the boys' age in THE ADVENTURES OF TOM SAWYER.
So much so that I've wondered if he was deliberately aiming at the most
diverse possible audience of children.  In the opening pages Tom seems
about 10, or maybe younger.  In the closing scenes he seems about 15,
and the narrator tells us he's nearly a man.  Yet the action spans one
endless summer.

Mark Coburn