User blog comment:Semanticdrifter/Updates to COPPA/@comment-15705679-20130809184920/@comment-413936-20130809191612

Most of it is that there's a fair amount of damage that can (and has) occurred from personal data of children. As far as the law, 13 is the dividing line on many things as (best I understand) that's where the child begins to be transitioned out of the "parents deal with everything" mentality towards future stuff such as driving and being considered an independent, legal adult. The main issue is that to use (edit, post, etc.) almost any website, the user's IP address must be logged, and that's now considered to be protected personal data. With creating an account, there's always a certain amount of personal data that's required to set it up (i.e. email address, sometimes name), though there are a handful that don't require any form of personal info. Due to how Wikia functions, some degree of personal data is necessary.

But on a similar note, there are also age restrictions on driving permits, driver's licenses, buying cigarettes, and alcohol in the US. The ages were picked based (at least in part) on trends of maturity, responsibility, and size - though to be frank, some have the maturity levels much earlier and some seem immature enough they should be barred for life : P