I wanted a more advanced user script that didn't just randomly set your age to somewhere around 1950s. This one sets your month to June, day to 22, and year to 1980. But you can easily change it by opening the script up and changing the 3 values I marked for you.
//Month var m = 6; //Day var d = 22; //Year var y = 1980;
Simply change month to the number month, 12 being December, 1 being January. Day to anything in that month. Year, anything that the site allows, currently between 1900 and 2014.
Be warned, fake values or invalid values may cause the script to fail. This should make age check pages almost unnoticable, as they will blink by. This will also not work in steam browser, as you can't install scripts there. This has been tested in firefox and chrome.
Update 1.4; You can now have it randomize your birth date. While I recommend you use your real date, you can set line 13 to True, and it will override whatever is set as the date, and randomize it.
Update 1.5; Added single quotes around False/True, added additional equals in if/else statement, added a missing semicolon. Pointed out by dk5.