That is a very interesting and valuble insight.
As someone who works with a small team (For a 50,000 member group) and is constantly busy - there is no time for accepting requests. Furthermore, regarding accepting requests, how can you tell if a user is underage or immature. I may be missing some information here, but surely manually accepting people is time consuming and would require frequent moderation. I still stand by the fact that people under the age of 13 practically have no use for group walls. Removing the feature for them would prevent them delving into groups like mine and yours, with tens of thousands of different aged people - and characters.