If you want to get old-school with your 'Sonic' experience, you'll have to earn it.
As easy as it is to rag on Sega for their continued failure with the 'Sonic' games, 'Sonic Generations' actually looks like 'Sonic' done right. You get to relive some classic levels with a full HD and, best of all, you can choose between 2D and 3D versions, depending on whether you want the classic experience or prefer the newer games.
If going back to 2D 'Sonic' games in high definition isn't old school enough for you, Sega has packed in a way to take the game way back to its roots. The entirety of the very first title in the series, 'Sonic the Hedgehog,' is playable in the same classic sprite based style as on the Genesis way back in the day.
In order to play it though, you'll have to do some work. Going through the game will earn you points that you can redeem for goodies in the Skill Shop. Collect enough and you can unlock a Sega Genesis Controller that will give you access to the option to play through the original game.