According to Wikipedia, the initial release of MySQL was 23 May 1995. This places it at 20 years old young tomorrow.
2015 is actually a year of anniversaries. As well as MySQL reaching the big two-oh, we are also celebrating 10 years of InnoDB under the stewardship of Oracle, and five years of MySQL @ Oracle.
Please make sure toast to MySQL this weekend!
(Photo from Percona Live community dinner 2015.)