Don't really know anything about the law but I do like to think I have an ounce of common sense. If I was to trim a payroll I would have to look at the highest ones first, right? Reason would say that it takes time to work your way up the ladder and therefore a person naturally increases in age. Therefore if you trim your payroll you are going to get a higher percentage of people in the older age bracket, right? So therefore conscientiously or unconscientiously you are discriminating against older people. Sure, you can say that a few young people get paid handsomely but the percentage is smaller, right?
I have seen it and have been a victim of age discrimination but like you said it is hard to prove and the companies have the best $$$ lawyers at their disposal.