A 12-year-old can quickly mature to meet the sites' minimum age requirements, generally 14, while an adult looking to chat with teens can virtually shed several years.
With heightened concerns over sexual predators lurking at so-called social-networking sites, state attorneys general have called for such communities, particularly MySpace, to improve age and identity checks.
If only it were so easy, experts say.
"We're all just grasping for solutions," said Anne Collier, co-author of the forthcoming "MySpace Unraveled: What It Is and How to Use It Safely." "We haven't fully researched it and thought about all the implications."
News Corp.'s MySpace has met with several companies on technologies to verify ages, but it has yet to find an effective one, Michael Angus, general counsel of News Corp.'s Fox Interactive Media unit, told Congress last month.
In an interview, MySpace's safety czar, Hemanshu Nigam, said any technical solution must be part of a set that includes education and cooperation with law enforcement.
"As we progress in our evaluation of what's best out there, you're going to see many different things coming," he said.
Parents, school administrators and police have become increasingly worried that teens are finding trouble at social-networking sites, which provide tools for messaging, sharing photos and creating personal pages known as profiles. The aim of such sites is for users to expand their circles of friends.
Southern California-based MySpace has gotten the brunt of the complaints, given its leadership with more than 94 million registered users — about 20 percent of them under 18.
In June, the mother of a 14-year-old who says she was sexually assaulted by a 19-year-old user sued MySpace and News Corp., seeking $30 million in damages. The lawsuit, filed in a Texas state court, claims the 19-year-old lied about being a senior in high school to gain her trust and phone number.
MySpace has recently implemented policies designed to better separate kids from adults. Among the changes, adult MySpace users must already know a 14- or 15-year-old user's e-mail address or full name to initiate contact or view a profile containing personal information.
However, because age is self-reported, as it is at similar sites, adults could simply sign up as minors.
There are tools to verify age, but they work best for porn, wine-sales and other sites meant for adults only.
A credit card, for instance, could demonstrate that a user is of age, notwithstanding a teen's ability to "borrow" a card from Dad's wallet.
More robust techniques like those from IDology Inc. and Sentinel Tech Holding Corp.'s Sentry check addresses, birth dates and other information users provide against public databases, such as voting and property records.
But many social-networking sites cater to both adults and teens — and teens can be difficult to verify.
Minors "do not possess as many unique identifiers as adults do," said Adam Thierer, a senior fellow with the Progress and Freedom Foundation, a technology think tank that shuns government regulation. "They are not voters yet. They don't have home mortgages or car loans. Most don't have drivers licenses until they are 16."