“We gave more kids an opportunity to have a second lease on life,” she said.

It was in 1993 that Colorado passed its laws giving prosecutors broad authority to charge young offenders as adults. Ms. Dvorchak said the system became little more than a “plea mill,” with juveniles represented by lawyers who worked chiefly with adult defendants, and who urged many of them to accept plea bargains. “No one had any independent review of whether that was appropriate for the child,” she said. Juveniles who lost at trial faced mandatory sentencing guidelines and felony convictions that could ruin their career prospects.

Ms. Dvorchak’s initial attempts to change the system failed. Legislators, she found, had been convinced by prosecutors that the minors they chose to try as adults were the worst offenders — “serial killers and rapists.” Her group produced a report that showed that 85 percent of the 1,800 cases over a 10-year period involved middle- to low-level felonies like robbery, assault and burglary; only 15 percent involved homicides and 5 percent first-degree murder.

When Ms. Miera joined forces with Ms. Dvorchak, her compelling story about her brother and the new data from the advocacy group helped change lawmakers’ minds just as other states were changing their laws. They found an ally in State Representative B. J. Nikkel, a Republican, who assembled a bipartisan coalition that overcame prosecutors’ objections. “It was the hardest-fought bill I ever worked on,” Ms. Nikkel said.

She said the old system “put too much power in one place — the district attorneys,” who, she argued, were biased in favor of racking up convictions. Her legislation is about “giving a role back to a judge to actually look at the information from both the prosecution side and the defense side, so they can weigh this out.” Her work, she added, was influenced in part by groups like Right on Crime, a conservative organization that highlights the potential for budget savings in criminal justice reform.

The state’s district attorneys are unhappy with the change. Peter Weir, a prosecutor in Jefferson and Gilpin Counties, called the law “abysmal,” and a step back from a system for young offenders that he said was working. Prosecutors do not follow a bias to convict people, he said, and prosecutors develop acute insight when it comes to juvenile cases — better, he said, than most judges. “This is a function of experience and is a function of years and years of weighing one case against similarly situated cases,” he said.