TeX.SE is listed on Area 51. However, the right-hand column only references that "this site launched ... 8 years ago."
was proposed on June 2, 2010;
had a commitment start on June 22, 2010;
went into private beta on July 26, 2010; and finally
went into public beta on August 2, 2010
before launching. When was the actual launch day/graduation?
It's not possible to establish this date purely based on questions on the site, since questions asked during the establishment phase on Area 51 - private and public beta - eventually end up on the site and therefore precede the launch date. As an example, the first question on TeX.SE (by reference in the URL as
/questions/1) - Automatically define macros in LyX documents - is dated July 27, 2010.