SAN FRANCISCO — Just how hard can it be to verify the age of a person online?

After all, privacy experts have been complaining for years about how much advertisers know about people who use the Internet.

The answer, it turns out, is very hard. Despite attempts by privacy advocates, academics, law enforcement officials, technologists and advertisers to determine a person’s age on the Internet, the reality is that, online, it is extremely difficult to tell whether someone is an 11-year-old girl or a 45-year-old man.

The question arose last week after Skout, a mobile social networking app, discovered that, within two weeks, three adults had masqueraded as teenagers in its forum for 13- to 17-year-olds. In three separate incidents, they contacted children and, the police say, sexually assaulted them.

In response, Skout suspended its app for minors, appointed a task force of security specialists to investigate and find solutions and said it would not resume the service until it could find a better way to vet users’ ages online.