Voluntary packages are fine, it's still the employee's decision regardless of incentives. I've lead a few forced layoff processes for my department. I never designed policies, but had to implement them. Legal and compliance were crystal clear on lines we could not cross and age was definitely one of them.
OK, so looks like you worked for one of the good ones. I'm giving you three very big company examples and backing up Dave's as a 4th. Perhaps it was your experience that was the exception? Was your company a $50billion + corp with a boatload of attornies on staff and billions in reserve to deal with stuff? Or a smaller company that couldn't afford to get it butt sued ?
BTW, sometimes just sometimes the alternatives to taking the deal are not so desirable? There are ways around everything. You should play around on Google to get some additional insights on the subject and get the general feelings from legal folks on the subject. What's THE LAW vs. Whats Practical and common practice.