Today, June 23, one of the world's most famous mascot video game turns 20 years and to wish him a happy birthday, Sega has released the demo on the Marketplace of Sonic Generations that will allow us to test the level of opening of the very first Sonic the Hedgehog's Green Hill Zone. The content is, as always, also append the web and will occupy 652.54 MB of space on your hard drive.