SAN FRANCISCO – A law that barred most offenders under 16 years old from being tried for crimes as adults does not violate the California Constitution, the state’s highest court decided Thursday.

In a unanimous decision, the California Supreme Court upheld a 2018 state law intended to end one of California’s tough-on-crime measures that helped lead to overcrowded prisons and defied scientific consensus on brain development of juvenile offenders.

Prosecutors challenged the law, arguing the Legislature usurped its authority in amending Proposition 57, approved by voters in 2016. The measure required prosecutors to start criminal proceedings against minors in juvenile court but allowed prosecutors to transfer some to adult criminal court.

Two years later, the Legislature amended the law to prohibit minors under 16 from being tried as adults.

Justice Joshua P. Groban, writing for the court, said the new law was a valid amendment of the ballot measure.

“The amendment is fully consistent with and furthers Proposition 57’s fundamental purposes of promoting rehabilitation of youthful offenders and reducing the prison population,” Groban wrote.

The court decision was a significant victory for advocates of criminal justice reform.

Sue Burrell, policy director of Pacific Juvenile Defender Center, which co-sponsored the 2018 law, praised the court’s ruling for restoring the law to where it was years before “draconian” measures passed to crack down on crime.

“It makes no sense for us to send young people to the adult system for something they did when they were in ninth or 10th grade,” Burrell said.

New research has found that prisons and kids don’t mix. Researchers say that adolescents have lower impulse control, greater mood swings and suffer long-term damage from the kind of prolonged isolation they endure in prison.

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