If you've had Facebook for years and have posts from quite a few years back, you'll look back at them and think "oh my god, I can't believe I said that". This is definitely a problem for me and I'm sure is a problem for a lot of others.
Unfortunately there is no complete way to hide all these past posts. The only thing you can do is limit them to your Facebook friends. I don't see this as a problem because most of my Facebook friends know me very well, and I know they won't waste their time to just look at my old posts - you're sad or a freak if you do.
How To Hide Old Posts
1. At the top right hit the arrow button next to the "Home" button, then go to "Privacy Settings".
2. Then scroll down and look for the "Limit the Audience for Past Posts".
3. Then hit "Manage Post Post Visibility". You will be presented with this window.
4. Then just hit "Limit Old Posts".
Remember, it will only limit all your posts to your Facebook friends. Hopefully soon, Facebook will enable you to completely hide previous years from everyone, that's a feature I definitely want.