Penalties along with interest on top will be applied to those who owe back taxes and this amount can be in the thousands of dollars. The definition of delinquent income taxes, per the IRS, is any tax return that is shows a US postmark that is after April 15th of that year, and this is if an extension was not requested.
After three years, the IRS will keep any return money that you were due. Once past that date, legally, the government is not required to pay any refund that is owed to you. And this goes for any amount, even if it is thousands of dollars. If you fail to file taxes, know that they will find you. Even if you have not heard from them in years, one day, they will arrive.
They have their ways of collecting from you and one way is wage attachments, bank account levies, revocation of permits or licenses, they can seize your property and they can refer your name to a private agency for collection. Never sign any form from the IRS until a tax professional looks them over. Only they can give advice on your options.