Age is a big no-no. Never ever ask an employee or potential employee how old he or she is. It will get you into hot water. The Age Discrimination in Employment Act of 1967 (ADEA), protects individuals who are 40 or older from being discriminated against in the workplace in favor of younger employees. The only time you are able to ask an employee his or her age is in general terms on an application. You are only allowed to ask if he or she is over the age of 18. That's it.
Don't get into the details of a potential employee's personal life. Although it is not illegal to ask if someone is married, it can be the basis for a lawsuit. If the person happens to be in a same-sex relationship or marriage and you ask this question- then not hire the employee- it can be construed as discrimination. Also, if you ask this of an employee and then an employee gets passed up for a promotion- it can also equate to a lawsuit. Why? Well, the employee can say he or she was passed up because you thought their employment and or marriage wouldn't survive a transfer to another location or additional hours- even if this reason has nothing to do with the actual facts why someone was overlooked for a promotion.
Believe it or not- it is ILLEGAL to ask if someone is a U.S. Citizen! Yes, this is true. Citizenship and immigration status cannot be used against a potential employee during the hiring process according to The Immigration Reform and Control Act of 1986 (IRCA).Employers must wait until after a job offer had been extended to require a worker to complete the Employment Eligibility Verification (I-9) Form and submit documentation that proves identity and employment authorization. However, you can avoid potential issues by asking if someone is authorized to work in the US- which is perfectly legal.
There are other questions that you should shy away from too. You can ask me directly on Twitter or your in-house attorney.