If your parents only income is social security benefits, they do not have a filing requirement. Social security benefits do not count as income. If they file a joint return, you cannot claim them as dependents.
You
can claim your parents as dependents if you provide more than half of
their support in 2018 and they had income of less than $4,150 (not counting social security) each.
https://turbotax.intuit.com/tax-tips/family/steps-to-claiming-an-elderly-parent-as-a-dependent/L34je...
https://blog.turbotax.intuit.com/tax-deductions-and-credits-2/family/can-you-claim-a-parent-as-a-dep...
