Those would be listed out in the Community Standards document, which, fair or not, doesn't list age at all as one of its protected categories. To expand beyond that, they don't actually have protected categories, just community guidelines. The closest they get to even mentioning an age-related ban is emphasizing everyone on the adult grid must be 18 or over; and that residents are:
...entitled to a reasonable level of privacy with regard to their Second Life experience. Sharing personal information about a fellow Resident --including gender, religion, age, marital status, race, sexual preference, and real-world location beyond what is provided by the Resident in the First Life page of their Resident profile is a violation of that Resident's privacy. Remotely monitoring conversations, posting conversation logs, or sharing conversation logs without consent are all prohibited in Second Life and on the Second Life Forums.
I also find this highly relevant, in relation to Representative Mark Kirk and his shrill 'warnings' against Second Life and other "online networking" sites--"Again we see that one of the most important parts of anti-sex hysteria is confusing minors looking for sex on the internet, some of whom may be of legal age, even if they are not allowed to be in Second Life, or viewing porn, with sexual predators.".
However, none of this changes anything--the main concept is still, SL's children won't be allowed to build.