I see this question all the time so I thought I'd look it up. This article https://www.thebalance.com/is-it-illegal-to-ask-for-a-job-applicant-s-date-of-birth-2059528 says basically that there are times that they can ask and times that they can't. It's legal to ask for it during the job screening process (which, what?! WHY, isn't that when they'd weed out people and COMMIT the age discrimination?) but apparently a lot of places ask for it so that they can do background checks. Basically if you think you've been discriminated against you should contact the EEOC (they tell you how to do that in the article).