Asking Applicants their Date of Birth
Date of birth isn't a question that is clearly illegal, like asking someone if they're in a union or if they're an American citizen. But asking an applicant to provide his or her date of birth/age is not usually wise and should be avoided. (After an applicant becomes an employee, i.e., after he or she is hired, asking for date of birth/age is still not a great idea, unless that information is needed for a pretty good business reason, like for benefit information.)
Source: Epstein, Becker & Green, New York, May 5, 1998.