In one case, the girl ran away from a group home upstate and went to work for the first man she met when she got off the train at 42nd Street. She was 12.

In another, the girl went home with an older guy she met aboard a subway car, only to work for him later from the basement of his grandmother’s Bronx row house, which had been turned into a makeshift brothel. She was 13.

“Sarah,” a 16-year-old former prostitute from Queens, said she first met her pimp the night she ran away from foster care, walking aimlessly around the streets of Jamaica past midnight with nowhere to go. She was 11.

While formal statistics tracking patterns of child prostitution are not kept by local enforcement agencies, and official studies on the subject are seldom conducted, one trend that experts in law enforcement and advocacy circles agreed on is that the average age of girls entering “the life,” as many put it, is decreasing.

The average age of New York children entering prostitution has now fallen to between 12 and 13, experts said. Several years ago, a national average age of 14 was reported by the National Center for Missing and Exploited Children, a nonprofit agency that also reported that some children enter prostitution as young as 9.

In New York, experts said the average age has been steadily decreasing in mostly low-income neighborhoods in Brooklyn, Queens, and the Bronx. Arrests for prostitution, a misdemeanor offense punishable by up to a year in jail or a detention center for minors, have also been up by nearly 5% over the last year across the city, according to police statistics.

“Business is booming, and the girls are getting younger and younger,” the executive director of Girls Educational and Mentoring Services, Rachel Lloyd, said. GEMS is a Harlem-based nonprofit organization that offers transitional programs to teenagers and young adults working in the sex industry.

The spike in child prostitution and the decrease in the age of practitioners can be attributed partly to fallout from the city’s crack epidemic of the early 1990’s, which led to broken families, Ms. Lloyd said, and to tough laws on sentencing drug dealers, a “glamorization” of pimps in popular culture, and “degradation of young women’s bodies” in magazines and music videos.

Ms. Lloyd, herself a former prostitute, has been collaborating on proposed legislation with a veteran Legal Aid attorney who specializes in child prostitution cases, Kate Mullen, along with another advocate, Margaret Loftus, of the nonprofit Juvenile Justice Project of the Correctional Association of New York. Their bill would alter the state’s penal code and change the status of girls arrested for prostitution under age 18 from “criminals” to “victims.” In essence, the proposed law would deter prosecutors from sending prostitutes under 18 to juvenile detention centers, and ensure that sexually exploited children can find help at emergency shelters.

Ms. Lloyd said current laws that penalize minors arrested for prostitution are archaic and contain double standards. If a 14-year-old female is alleged to have consenting sex with a man over 18, she said, laws punish the man with a criminal charge such as endangering the welfare of a child. If the 14-year-old charges the man for sex, the teenager is held criminally responsible. Instead, the law should treat the teenager as a victim, Ms. Lloyd said.

While some prosecutors said they don’t look to punish teenage prostitutes with criminal charges, Ms. Lloyd said many teenage prostitutes are sent to juvenile detention centers because they are considered high-risk after running away from group homes. Others are often sent to prisons for adults because they lie about their age.

When questioned by police, Ms. Lloyd said, many teenagers lie and claim they are over 18, and they look older and have fake identification to bolster the claim.

Sarah, who now works for Ms. Lloyd’s organization as a part-time administrative assistant, said she had been arrested by vice cops for prostitution 13 times before she turned 15. She lied about her age to police because her pimp told her she would be released sooner as an adult. After several arrests, she said, the pimp told her to lie about a drug problem and request treatment, so she could be released sooner. After failing to attend the required drug-treatment programs and getting arrested again, Sarah said, she spent eight months at Rikers Island.

A Police Department sergeant assigned to the Juvenile Crime Squad said it is common for teenage prostitutes to lie about their age and for police officers in local precincts to charge them initially as adults. Another flaw in the current system, the sergeant – who requested anonymity. citing police departmental rules – said, is that teenage prostitutes who work a “track” are often charged with loitering, which is a violation, a low-level offense that allows juvenile offenders to sidestep Family Court and return to their pimps.

The Juvenile Crime Squad sergeant also reported ages of child prostitutes entering “the life” to be decreasing, and estimated the average age to be 12 or 13, and the average age they are arrested to be 14 or 15.

Ms. Mullen, the Legal Aid attorney, said that her caseload of prostitutes under 18 has tripled over the past two years, and of the 20 or so cases she now has pending before Family Court, the bulk of those facing charges are 13, 14, or 15. Most, she said, had histories of sexual abuse as children and began relationships with their pimps long before being arrested.

The pimps, too, are getting younger, she said. “The pimp of today is not the middle-aged man you are thinking of with the funny hat and the platform shoes,” she said. “The pimp of today looks like someone else’s boyfriend. He doesn’t drive a flashy car. He’s unassuming. Maybe he is a guy who figured out it’s less risky and more lucrative to deal girls than drugs.”

A Queens assistant district attorney and bureau chief for special proceedings, Tony Communiello, who oversees prostitution cases, called the decreasing age of child prostitutes “a scary trend” fueled by quick money. It’s a trend he first noticed a few years ago when an undercover police officer picked up a woman dressed like a prostitute and soliciting sex from cars passing through one of the city’s most well-known tracks, Queens Plaza.

She was four days past her 12th birthday, Mr. Communiello said. A runaway, she was not charged with a crime and was returned to her parents.

Another recent prostitution case he handled is illustrative of the problem, Mr. Communiello said.

In January, the police charged a 30-year-old man alleged to have raped and forced three runaways, ages 16, 15, and 13, to work for him as prostitutes, having sex with as many as seven men a night for $100 an act, in places such as parked cars and “hot-sheet” motel rooms. On March 10, the alleged pimp pleaded guilty to promoting prostitution and faces a potential 6-year prison sentence.

Meanwhile, he said, the teenagers have split in different directions. The 16-year-old was returned to her parents; the 15-year-old was sentenced to mandatory counseling and probation; the 13-year-old was placed in a city shelter – and quickly ran away.

While Mr. Communiello has not seen the language of the proposed bill, he welcomed the idea of more specialized social services for teenagers coerced into prostitution.

“There’s a lot of troubled kids out there,” he said.