ELKHART, Ind. — Until one day in December, Zachery Anderson was a typical 19-year-old in a small Midwestern city.

He studied computer science at the local community college. He lived with his parents and two younger brothers in a sun-filled home on the St. Joseph River, where framed family photos hang from the walls and a pontoon boat is docked outside.

And he dated in the way that so many American teenagers do today: digitally and semi-anonymously, through apps where prospects emerge with the swipe of a finger and meetings are arranged after the exchanges of photos and texts.

In December, Mr. Anderson met a girl through Hot or Not, a dating app, and after some online flirting, he drove to pick her up at her house in Michigan, just miles over the state line. They had sex in a playground in Niles City, the police report said.

That sexual encounter has landed Mr. Anderson in a Michigan jail, and he now faces a lifetime entanglement in the legal system. The girl, who by her own account told Mr. Anderson that she was 17 — a year over the age of consent in Michigan — was actually 14.

The case came to the attention of the police after the girl’s mother contacted them, concerned about her whereabouts. They were at her home when the girl returned, according to The South Bend Tribune. A few weeks later, the paper said, the police visited Mr. Anderson, who cooperated and, in February, turned himself in. He was arrested and charged and, after pleading guilty to fourth-degree criminal sexual conduct, was sentenced to 90 days in jail and probation.

As an Indiana resident, Mr. Anderson will most likely be listed on a sex offender registry for life, a sanction that requires him to be in regular contact with the authorities, to allow searches of his home every 90 days and to live far from schools, parks and other public places. His probation will also require him to stay off the Internet, though he needs it to study computer science.

Some advocates and legal authorities are holding up Mr. Anderson’s case as the latest example of the overreach of sex offender registries, which gained favor in the 1990s as a tool for monitoring pedophiles and other people who committed sexual crimes. In the decades since, the registries have grown in number and scope; the nearly 800,000 people on registries in the United States go beyond adults who have sexually assaulted other adults or minors. Also listed are people found guilty of lesser offenses that run the gamut from urinating publicly to swapping lewd texts.

As Mr. Anderson’s defenders see it, his story is a parable of the digital age: the collision of the temporary relationships that young people develop on the Internet and the increasing criminalization of sexual activity through the expansion of online sex offender registries.

“The whole registry is a horrible mistake,” said William Buhl, a former judge in Michigan who has publicly argued that laws governing registries ought to be relaxed. “I think it’s utterly ridiculous to take teenage sex and make it a felony. This guy is obviously not a pedophile.”

But once Mr. Anderson leaves jail in the coming week, he will be bound by the same restrictions that apply to more extreme sex offenders, tagged with a “scarlet letter” for life, as his father, Lester Anderson, put it.

“At the end of the day, he might be out of jail, but he’ll still be in his own jail,” his father said. “He has to walk down the street every day and think: ‘Am I too close to a school? Is there a child who’s close to me?’ ”

There are fledgling efforts in some states to change sex offender registries so that they do not include juveniles or those guilty of minor offenses. In California, the corrections department announced in March that the state would ease residency requirements for many sex offenders, allowing certain low-risk individuals to live in areas closer to schools and parks that were previously off limits. Many sex offenders have ended up broke and homeless, living in clusters under freeways because they are routinely rejected by employers and landlords, and because they are banned from living in so many neighborhoods that contain public places like parks.

Brenda V. Jones, the executive director of Reform Sex Offender Laws, an advocacy group, said cases like Mr. Anderson’s are common in many states. Frequently, a judge will give the lightest possible sentence, but cannot change the restrictions involving the offender registry.

“It’s like a conviction on steroids,” Ms. Jones said. “Being on a registry becomes a liability for employers, no matter how minor the offense was. Other people will say: ‘I saw your employee on the Internet. He’s a sex offender, and I will not come to your establishment.’ ”

Changing the laws has been a slow fight. “People talk about it, but when you actually try to introduce legislation, lawmakers start to get really nervous,” Ms. Jones said. “Because, oh, my God, we’re going to be soft on sex offenders.”

Mr. Anderson’s parents are fighting back on behalf of their son, saying that while they believe he made a mistake, his punishment is extreme. They have been joined by the girl, who is now 15, and her mother, who have also defended Mr. Anderson, appearing in a Federal District Court in Michigan this spring to ask a judge for leniency.

“I don’t want him to be a sex offender, because he really is not,” the mother said, according to court transcripts. Her daughter told the judge that she felt “nothing should happen to Zach,” adding, “If you feel like something should, I feel like the lowest thing possible.” The judge, Dennis M. Wiley of Berrien County District Court in Michigan, was apparently not swayed by their testimony. After Mr. Anderson pleaded guilty to criminal sexual conduct in the fourth degree, the judge declined to grant him a special status intended for young offenders. The status, under the state’s Holmes Youthful Trainee Act, would have spared him inclusion on the sex offender registry and erased the conviction from his record if he did not violate probation.

During a sentencing hearing in April, Judge Wiley criticized online dating in general and berated Mr. Anderson for using the Internet to meet women.

“You went online, to use a fisherman’s expression, trolling for women, to meet and have sex with,” he said. “That seems to be part of our culture now. Meet, hook up, have sex, sayonara. Totally inappropriate behavior. There is no excuse for this whatsoever.”

The prosecutor, Jerry Vigansky, did not oppose a Holmes Act sentence, but noted that it had not been applied to two similar cases in recent months.

For some reason, Mr. Vigansky told the judge in court, this generation seems to think it is “O.K. to go online to find somebody and then to quickly hook up for sexual gratification.”

“That’s not a good message to send into the community,” he said.

Mr. Anderson and his defenders say it is precisely that culture that makes it difficult to determine if a prospective sexual partner is under age, when introductions occur online and outside traditional social networks. It is also easy to misrepresent personal details, including age.

“When somebody impersonates an adult, that should be a factor,” said Scott Grabel, Mr. Anderson’s lawyer. “You could argue that he wasn’t negligent. I don’t think this kid should be labeled a sex offender: That outcome doesn’t do anybody any good.” Mr. Grabel declined to make Mr. Anderson available for an interview.

But Rick Jones, a state senator in Michigan and one of the authors of the state’s sex offender registry laws, dismissed that defense. The law requires people to be responsible for determining the age of their sexual partners, he said, and in a case like Mr. Anderson’s, the punishment seems appropriate.

“A 19-year-old knows that you have to be very careful, and you certainly should not be having sex with a 14-year-old,” Mr. Jones said in an interview. “In my opinion, society, over several decades, has become looser. People are meeting online, and that creates all sorts of problems. Now, people have all these crazy apps where you can locate people in your vicinity where people want to have a relationship. You should be very careful.”

He said he was not bothered by the terms of Mr. Anderson’s probation, which require him to stop using the Internet for five years.

“There are lots of jobs that don’t involve computers,” he said. “There are all sorts of trades. Truck drivers, welding. There are other opportunities.”

That kind of talk infuriates the Andersons. They said the terms of their son’s probation are unnecessarily severe and outdated, and would make it impossible for him to continue to attend college.

“No computer for five years, no smartphone? He can’t have an email address,” his father said. “To me, that’s wrong. That’s like taking away electricity or heat or gas to somebody, in today’s world.”

With their son’s release from jail set for Thursday, they were scrambling to find him a new place to live and satisfy the sex offender restrictions on housing. Their own house is less than 1,000 feet from a public boat launch, which is considered a public park under state law.

Since they have shared their son’s experience with local newspapers and elected officials, they said, they have received emails and Facebook messages from parents whose children have been punished for similar offenses and are now on the sex offender registry for life.

Their lawyer has asked to withdraw his client’s guilty plea so he can argue for a lighter sentence. A hearing is set for Aug. 5.

“A young person, they make one mistake and all of a sudden they’re classified as a loser for the rest of their life,” Mr. Anderson’s mother, Amanda Anderson, said. “This scenario should never result in jail time or a life of anxiety.”

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