Sounds bad, but assuming an arrest under 23 can happen any time between the ages of, say, 7 and 22, that's actually 2.7% in any given year, or roughly 1 in every 40 male children.
It almost certainly points to police officers arresting children en masse in order to discover the real perpetrator rather than any child criminality epidemic.
It's not the rate that makes it bad , but the effects. In other words, it's not that there is a crime epidemic or a major problem with youth, or that every year only 1 in every 40 male children get arrested but that over all their youth-time taken as a whole 40% will have been arrested. But even that is not the key point from the abstract. The key point - what makes it bad are:
1) "Criminal records that show up in searches can impede employment, reduce access to housing, thwart admission to and financing for higher education and affect civic and volunteer activities such as voting or adoption. They also can damage personal and family relationships."
2) "A problem is that many males – especially black males – are navigating the transition from youth to adulthood with the baggage and difficulties from contact with the criminal justice system,"
I'm going to go out on a limb and say that the arrests are not evenly distributed betweeen the ages of 7 and 22. In fact I'd guess that a statistically insignificant number of arrests occur before the age of 10, and that they cluster around 16-22.
It is probably higher than 2.7%. The 40% statistic concerns probability of arrest, not expected number of arrests. In plain english, if a kid got arrested twice, he'd just show up once in the 40% figure.
It almost certainly points to police officers arresting children en masse in order to discover the real perpetrator rather than any child criminality epidemic.