Skipping to chapter 19, Huck and Jim become "one with nature". They take in some criminals and Huck can tell that they are lying. This sense is acquired as we mature and experience life which has to do with "Coming of Age". As we grow up we deal with how people act when they are truthful and when they lie so we gain the sense of telling the difference between the two. Huck doesn't tell Jim to try and protect him which shows us that their bond is developing.
Chapters 20 and 21 don't really involve "Coming of Age". The criminals go around town and scam people out of money while Huck and Jim stay on the raft. Towards the end one of the criminals places a $200 bounty/reward for the capture of Jim. It seems that Huck and Jim can't catch a break.