Officials in Connecticut, Illinois and Vermont are even discussing raising the cutoff to 21, though this is more widely disputed. In New York, Gov. Andrew M. Cuomo has proposed raising the age for adult culpability to 18, but members of the Republican-controlled Senate have expressed concerns about transitional costs as well as public safety.

In Lafayette, La., Rob Reardon, the director of corrections for the parish, explained why he thought that both the state and the young offenders would be helped by treating 17-year-olds as juveniles.

“This is an adult jail,” he said, “and the outcomes for the young people just have to be terrible.”

On a recent day, the parish jail held one female and eight male 17-year-olds, awaiting adjudication for crimes that ranged from trespassing to murder.

When they spend a few weeks in his jail, Mr. Reardon said — and for those who cannot make bail, it is often 90 days or more — youths are expelled from school and never graduate.

The presence of 17-year-olds is also a major resource strain, he said. Inmates generally live in pods of 13 cells with double bunks, or 26 to a pod. According to a federal law (one that has frequently been violated in New Orleans), youths must be kept separate, so one entire pod was occupied by just the eight 17-year-old boys, who were tended 24 hours a day by a guard. The 17-year-old girl was housed alone in a unit with four cells.

The separation also means that the youngest inmates cannot take part in activities like kitchen work. In the youth pod in Lafayette, guards had brought in coloring books and jigsaw puzzles to give the boys something to do.