Deja Vu is caused by tons of unrelated information in your brain. You get a lot of sensory input throughout your life, that doesn't relate to any specific memory, and your brain stores it for later reference. With all the media, TV, Computer, Youtube, books, Magazines, etc, there is a lot of useless information out there. Sometimes, when you sense something, your brain tries to connect it to the "junk" information in your head. When it finds something that looks connected, it ties it together. Giving you the impression that you have seen this before, or experienced it before. As you grow older, this should happen less often.