http://liveweb.archive.org/http://www.nlm.nih.gov/medlineplus/news/fullstory_119873.html

By the time they're old enough to vote, roughly one in four Americans has had at least one criminal arrest.

By age 23, an estimated 33 percent -- and perhaps as many as 41 percent of young adults -- have been arrested at least once, excluding traffic violations.

The authors of a new study say their analysis is the first contemporary look at national arrest prevalence in this age group since landmark research in the 1960s.

"The percentages are not really all that different up to age 18," said lead researcher Robert Brame. "The biggest differences are from 18 to the early 20s, and most occur in the 19-to-22 age range."
...
The study didn't explain why arrest risk grew so much for young adults, but the authors suggested a couple of possibilities.

"The criminal justice system is more punitive today. In adult systems, in 1972 to 1973, there were 100 inmates per 100,000 people in the population. It's an increase of about five times more today," Brame said. "So we're mindful of that."

Another possible reason is that the transition from childhood to adulthood takes longer than it used to. "We speculate that society as a whole is going through a phase of extended adolescence," Brame said. "The phenomenon has a name, 'emerging adulthood.'"