As an employer there are limitations on what you can ask about a potential employee. However if the reference offers information then an employer is allowed to listen.
Legally in the US all your previous employer is allowed to say about someone who calls for a reference is to confirm your dates of employment and whether or not you are eligible for rehire. In reality that it not what really ever happens, because so much of this industry is based on connections and professional relationships, so people will share more than that.
As an employer in the US calling about a reference, you can't ask anything about a person's age (except that they are at least 18), family or marital status, why the person is choosing to relocate, about disabilities, or any of the other questions that you would not be allowed to ask a candidate in an interview.