I am 90 years old and my wife is 86. we only make $35000/year with social security and a small pension. Do we need to file taxes?

Yes. You are not likely to owe any taxes, but you always need to file a return. Under the Internal Revenue Code section 6651, you can face penalties for not filing your taxes, even if you do have an actual tax liability for the year. In short, always file, even if you are sure you won't owe anything.