what evidence is presented to esablished huck as a youth rather an adult

This mostly comes at the beginning with Huck not wanting to tow the adult line. He doesn't like church or school. He wants to explore, play and have adventures.

http://www.gradesaver.com/the-adventures-of-huckleberry-finn/q-and-a/what-evidence-is-presented-to-establish-huck-as-a-youth-rather-than-an-adult-59879/

My other answer that Jill pointed out to works with this.