If you are a minor, you cannot LEGALLY cosign or sign any loan/contractual agreement.
Yes, an employer can cosign for a student loan. Any one can cosign for a student loan if they meet the requirements.
The loan you cosign is going to count as a loan that you have. If you have the income to support that loan, plus yours, from a loan officer's point of view, you may be able to get a loan. Generally, though, cosigning is a bad idea for anyone.
Yes, someone on the social security can be able to cosign for a loan. The person cosigning the loan however has to have good credit regardless of his availability on the social security benefit.
Never cosign a loan. While I agree that one should NOT cosign. cosigning can hurt or help. Remember that if they do not pay you have to. Cosigning will affect your credit and count towards your debt to income ratio and show as an open joint auto loan. You might be turned down to get your own auto loan without a cosigner if you cosign.