What evidence is presented to establish Huck as a youth rather than an adult?

It is understood that Huck is a youth from Tom Sawyer. In Huck, we know that he is young because he is adopted by the Widow Douglass, after which, his Pap shows up to claim custody ( he's after Huck's prize money). Also Huck's mannerisms and ponderings are much like a youths.

In addition, when we first meet Huck, he has no desire to tow the adult line. He doesn't like church or school. He wants to explore, play and have adventures.