DO I have to file federal income tax after age 70?

My mom is 88 years old and is still paying outrages federal income taxes. I have always heard, that at age 70, you could earn as much as you want, without being taxed. She has a little farm, which pays her about 20,000 a year, and she has a very small pension, and a very small social security, since she is what they called a notch baby. Could you give me some information on how she should be taxed?