If I’m 91 and only have retirement income, do I still have to file taxes?

The requirement to file is not based on your age.  It is based on how much income you received.   You say that you have retirement income, but did not mention the amount of that income.

Who has to file?

http://www.irs.gov/uac/Do-I-Need-to-File-a-Tax-Return%3F

Run this IRS app

https://www.irs.gov/help/ita/do-i-need-to-file-a-tax-return