The only time you don't pay social security taxes are:
1. you are under 21 and you work for your parents
2. you are under 18 and you are a household employee for someone else working in their home (like a babysitter or aide of some kind)
3. you are a student, working for your school in a position that is related to your education (such as a college student who gets paid for working as a teaching assistant).
4. you work in certain specialized jobs where the job or your union provides a better retirement benefit than social security, in which case you pay dues for that instead.
Your social security withholding goes into the social security system and builds credits for your future retirement or possible need to go on disability if something unfortunate should happen to you.
This answer has been verified for accuracy by an Intuit expert employee
