Based on the Equal Employment Opportunity Commission (EEOC), it is illegal to discriminate for employment and pay of an employee, based upon: age, gender, marital status, race, color, religion, sexual orientation, pregnancy, or disability. You simply must be capable of doing the job well.
Unfortunately, employers notice such factors during an interview. Someone may inadvertently ask if you have a child in conversation. Or mention another of the areas listed above. At that point, you may want to consider if this would be the right employer for you, in light of what they are asking. Are they familiar with the laws? If they are in Human Resources, I would not be comfortable entrusting this person with my confidential information!
However, it is possible that the interviewer simply does not know any better. If that is the case, I would ask how the point in question (age/parenthood/gender/marital status) would reflect on my capabilities as an employee. Perhaps, the interviewer is concerned about travel. You can share that you are able to travel the required amount. Dig deeper and identify the issue. See if there is more to the question. If not, that may just not be the employer for you.