JEFFERSON CITY, Mo. (AP) — The U.S. Supreme Court's ruling against mandatory life sentences for juveniles should also apply to older teenagers, a Missouri man imprisoned for killing his grandparents at age 19 argued to the state Supreme Court on Wednesday.

David Barnett, a suburban St. Louis man who now is 43, was convicted of first-degree murder and sentenced to life in prison for the 1996 murders.

His public defender, Rosemary Percival, told Missouri Supreme Court judges during arguments that Barnett had poor impulse control because his brain was still developing at that time.

She said if a judge or jury was able to weigh his age, the “horrific” physical and sexual abuse he suffered as a child and other factors, they would give him a less harsh sentence.

“At the stroke of midnight on the defendant's 18th birthday, life without parole becomes mandatory with no consideration of the defendant's character or circumstances," she told judges. “There’s no middle ground.”

The U.S. Supreme Court ruled in 2012 that teens should be treated differently to adult offenders at sentencing because they're less mature, prone to manipulation and capable of change. The court found that all but the rare juvenile lifer whose crime reflects "permanent incorrigibility" should have a chance to argue for freedom one day.

Percival told Missouri Supreme Court judges that the same arguments apply to older teenagers.

“The understanding of adolescent development is that nothing magical happens on one’s 18th birthday,” said Josh Rovner, a senior advocacy associate at The Sentencing Project. “So there’s been a great deal of discussion about the proper response to 18-, 19-, 20-year-olds and even older.”

The Sentencing Project is a Washington-based nonprofit that opposes sentences of life without the possibility of parole.

Missouri is one of just five states — along with Georgia, Michigan, Wisconsin and Texas — that begins adult court jurisdiction at age 17 rather than 18.

But Missouri and Michigan have enacted laws that will raise the adult court age to 18 starting in 2021. Missouri’s law is contingent on the Legislature appropriating money to provide the expanded juvenile services.

The issue of when teenagers and young adults should have a chance to argue for leniency based on their age could ultimately fall to Missouri lawmakers. Several Missouri Supreme Court judges on Wednesday questioned whether it's their role to overturn mandatory life sentences for older teens, or if it's up to the state Legislature to change the law.

Judge Laura Denvir Stith told Barnett's attorney that she made sound arguments on changing the policy, but asked: "Is it unconstitutional, as opposed to a policy?”

An attorney for the state argued that it should be left to the U.S. Supreme Court to revisit the age cutoff for mandatory sentences.

Judges did not indicate when they might rule.

___

Associated Press writer David A. Lieb contributed to this report.