The state’s oldest teenagers would still be considered juveniles under proposed legislation announced Tuesday that would raise the age to 20 for adult prosecution.

Sen. Nancy Skinner, D-Berkeley, said the measure would amend state law by designating 18- and 19-year-olds as juveniles in criminal proceedings, ensuring they get support services for youth rather than punishment for adults.

“We have 21 as the age for alcohol. We have 21 as the age for tobacco,” Skinner said. “The research definitely shows that there’s an age difference in things like impulse control.”

While there should be consequences for young people who commit crimes, processing older teens in the juvenile system ensures they will receive appropriate counseling, treatment and other services, she said.

Exactly how that would work, or what it would mean for county juvenile justice systems, is unclear. Juvenile offenders are processed at the county level and can be placed in juvenile halls or, for the most serious offenders, in state youth prisons.

The proposed legislation comes amid a long-term drop in youth crime that has left almost every juvenile hall in California more than half empty — a trend The Chronicle explored in its Vanishing Violence series.

But as the population in custody has declined, the amount of money spent on many facilities has remained flat, causing the annual cost to house a youth to skyrocket. The Chronicle found that some counties are spending as much as $500,000 a year per child. San Francisco’s Board of Supervisors voted to close the county’s juvenile hall by the end of 2021.

So far, Skinner’s legislation, SB889, is a placeholder bill and lacks specifics.

The state senator said she will work closely with juvenile justice reform advocates to suss out the details, as well as the California Probation Officers Association, which in November proposed raising the state’s adult prosecution age to 20.

“This is a reform whose time has come,” said David Steinhart, director of the Commonweal Juvenile Justice Program, which advocates for alternatives to youth incarceration. “It will improve public safety by putting thousands of California’s youth back into education and on job tracks that are blocked when they are processed as adults.”

Some experts, however, question how such a major change can be implemented without overwhelming juvenile courts and probation caseloads. Others, including Steinhart, worry the change would put more young people in California’s juvenile halls and say the legislation needs to address such concerns.

In 2018, there were 17,200 arrests for felony crimes of young people ages 10 through 17 in California and roughly 14,400 arrests of 18- and 19-year-olds for similar offenses, according to state Department of Justice data. Moving those young adults — and others accused of less serious offenses — into the juvenile system would require major planning and changes, Steinhart said after the state probation chiefs’ recommendation in November.

Overall, there were about 38,000 felony and misdemeanor arrests of young adults ages 18-19 in 2018, which could increase the caseloads in juvenile court by 82%, according to the Center on Juvenile and Criminal Justice advocacy group.

In addition, there are currently about 1,000 18- and 19-year-olds admitted to state adult prisons each year, a number that would more than double the number of juveniles in state youth prisons.

Meanwhile, several states plan to raise or are considering raising the maximum age of their respective juvenile courts’ jurisdictions.

Vermont was the first to raise the age of juveniles in criminal court to over 18, and the measure will be phased in over the next two years to ultimately include all those under age 21.

The goal is to make sure kids are treated like kids, said Alameda County Public Defender Brendon Woods, who supports Skinner’s bill.

“Under California law, teenagers can’t buy cigarettes, beer, or even rent a car, yet they can be sent to prison for the rest of their lives,” he said. “When a young person gets in trouble, they need our help. They don’t need to be locked in a cage.”

Jill Tucker is a San Francisco Chronicle staff writer. Email: jtucker@sfchronicle.com Twitter: @jilltucker