I'm no expert on this but I don't find anything in IRS publications that excludes low income employees (other than household workers) from social security and medicare withholding. See p. 23 of this publication for employers: <a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/p15.pdf">https://www.irs.gov/pub/irs-pdf/p15.pdf</a>
If you are self employed, you are not required to pay self employment taxes on income of less than $400.
