The IRS can ask for any years it wants, but usually only 6 years are required. Technically, failing to pay your past due taxes is a misdemeanor. However, the IRS is much more interested in collecting what you owe than sending you to jail. Non-filers who voluntarily come forward are rarely prosecuted.
Although the IRS doesn't usually send you to prison, you will be charged steep penalties and interest on taxes you owe. You might be able to persuade the IRS to reduce your penalties. You will have to prove you had reasonable cause for not filing your tax returns. Example of circumstances that the IRS might consider to be a reasonable cause include: the death of a family member, mental illness, alcoholism, bad advice from your accountant, or extended military service.
Another reason to file is that you might be able to recover a refund if it is within the last three years. If the IRS owes you a refund for the tax year, you won't be charged any penalties for failure to file a return.