Taxes can be filed back for up to ten years! When filing these back taxes you must keep in mind that tax evasion penalties could apply. After ten years the statue of limitaions on back taxes runs out. This is why you file your taxes as soon as possible.
For more information on how to get started when you file taxes for past years please read this article http://ez-online-tax-filing.com//what-to-do-when-you-have-not-filed-taxes-for-many-years.html