I agree that it's very difficult in this day and age to track down information once it's put in.
I think we're placing too much emphasis here on kids who abuse the Internet. There's more at stake than that: things such as personal health information, things that are put into databases as a result of credit card purchases, and data that companies have a lot of control over but store long beyond the time they should, especially in the case of young people. If they're identified as people under the age of 16, they should have the opportunity to have that information erased by the time they get older.
There are different categories of information. I agree that it's difficult to recover something once it's out on the web, but there's a lot of other information that we lose track of when we always focus on Internet-based information. Really, there's a lot of other information in databases that becomes stale-dated and no longer relevant, and should in fact be purged.