I’m not an expert at SEO just yet, I’m just presenting information I’ve found on SEO forums from people who seem to be very savvy in this area. From what I have read, Google is mainly concerned with commercial domains (.com), and don’t bother penalizing the other extensions. There are no known cases of .edu or .gov domains being sandboxed.
It does make sense to pick and choose domains if Google is concerned with stemming the tide of search engine spam and if they have deemed that the greatest portion of this spam comes from .com’s. Maybe they pick and choose some other domain extensions as well — .co.uk would probably be the next most popular commercial domain extension after .com.
The ageing filter appears to be a manual thing as many SEO people point out that your site will rank the first couple of weeks after it is indexed, then dissappear for 6-8 months no matter what you do. Add to that the evidence that sandboxed sites are released in groups, rather than one-at-a-time (if it was an automated process, they would likely be released based on a timestamp and appear to come out of the box one-at-a-time).