Seems like they're breaking the law to me, but it's not completely black and white. In the odd chance it would happen, wouldn't saying "20+ years of experience required", prevent anyone younger then 20 years from joining the company?
http://www.eeoc.gov/laws/types/age.cfm
Also, requiring experience is reasonable anyway since its easy to make a case about why it's required to do the work. I'd dying to here what kind of BS hand-waving argument could be constructed for why having too much experience prevents you from doing the work.
What you are not allowed do is discriminate based on criteria that are not part of essential qualifications for the job. And I for one can't conceive of one single thing that disqualifies, say, Jeff Dean, from holding that job over a candidate a few years his junior.