Ever tried dropping your age from your resume? There is no requirement to put it on. If you get into the interview and the interviewer has an, "Oh, you're not what we're after" moment you know why. In this country that is grounds alone to sue so most companies will give you a decent interview, at the very least.
The approximate age of the candidate is going to be glaringly obvious even without a date of birth. It's easy to infer that they're over 40 from things like graduating college in 1975, having 20 years work experience, etc.
Actually I'm currently in the prime of my hiring age, and it's not listed at all, but it's very easy to infer, particularly if an individual has dated their college history (I honestly don't recall if I did or not).
