TheEgg, I have no desire to fill out an app just for the heck of it. You do not have to prove that you are telling the truth, I never did not believe you. All I meant was that I have not encountered that, leastwise not that I recall. I guess I could just not be remembering it as personally, I do not see the problem of them asking that on a job application. However, if it is not on the application, it should not be asked in the interview unless the person appears to be a minor or the job requirements are such that one must be of atleast a certain age and the person is not obviously well over that age. If one is over 40 and that question is asked in an interview, but is not on the application, that would be a big no-no for the employer to ask as it could set them up for a discrimination complaint if they did not hire the applicant.
I guess I am confused because the initial question here was about them asking this question in an interview but if the question was on the job application, I do not see why that question would be asked as they already know how old you are. The only reason I can see it being asked in that case is if you look like you are under 21 and the person interviewing you does not have your application in front of them and they are considering what kind of position they can offer you. As Queen-Evie has mentioned, some positions do require you to be of a certain age so if you look under that, then they may be wondering if they can hire you for that position or if they would need to hire you for a different position because of you being too young for that position, or if they can even hire you at all.
If you are obviously over 21, I can see no justifiable reason why this question would be asked in a job interview. As for the date of birth being asked on the job application, one reason I can think of for some jobs is some employers will run a credit check on you before the interview and having the birthdate can help them determine if the credit check came back on the right person. That reason really would not apply for most retail jobs however, unless it was for a management position, in which case they just might do that. I think the reason for it being on the job applications you are talking about is they are just generic applications that were not really thought out well.
Regardless of when this question is asked, why does it matter to you if this question is asked? The interviewer will have a pretty good idea how old you are just from looking at you and the employer will at some point, know your exact age. Unless you feel you that you may be discriminated against, does it really matter that they know your age from the get-go? What is it that makes this question bother you?
Like Animal said, sometimes they ask things that seem off the wall just to see how you respond. I think if I was asked this question in an interview, I might just laugh and say something to the effect of, "old enough to do the job and young enough to do the job". I really doubt they would push it further but if they did, I would answer but I also might just ask them why they are asking.
I am hopefully interviewing for a position this coming week, it will be interesting to me now to see if it is asked. In my case, there would be no justifiable reason for them to ask me this question in an interview. I hope none of the jobs I will be applying for plan to do a credit check on me, I have placed a fraud alert on my credit report which means they might not be able to get it, oh-oh, I didn't think of that before now. It would stink if that was used against me, I have really good credit so if that matters to them, I would like them to get my report and now they can't and I really want a different job soon.