Another factor which will soon impact this, if it isn't already, is the rapidly changing nature of youth. Fertility rates have been dropping since 2009 or so. Average age of parents is increasing. Teen pregnancy on a long and rapid decline.
All of these working together means that each year the act of having a child is much more deliberate and the parents likely having more resources. Which in turn should mean fewer youth delinquency, which as the article notes is how most in prison started out.