Age has nothing do with whether you need to file a tax return. You might be thinking of this:
You need to file a federal return if half your Social Security plus your other income is $25,000 when filing single or head of household, or $32,000 when filing married filing jointly, $0 if you are filing married filing separately.
DO I NEED TO FILE A TAX RETURN?
https://www.irs.gov/help/ita/do-i-need-to-file-a-tax-return
Who has to file?
http://www.irs.gov/uac/Do-I-Need-to-File-a-Tax-Return%3F