It's also almost impossible to prove on both sides. Even if Google has insane numbers showing how many young vs old people they hire, it's totally possible that the younger ones are simply more experienced with the technologies/paradigms Google is looking for. A judge might not see it the same way though.
(See, e.g., the Kevin Spacey lawsuit for a recent example in the news)
Again, the question is not who would win. It's whether it is worth winning.
Let me use a strong amount of hyperbole to try to make this part of the point.
Imagine a 3 year old challenges you to a fight. They are completely and totally adamant about it. You certainly could win. Is it worth winning? Or would it be better to try to give them ice cream and get them to stop bugging you. Now imagine this fight will take 5-7 years :)
Despite what random comments on hacker news say, these are basically settled on a regular basis as cost of doing business.
If they were arbitration (heresy, i know) or some other cheaper/less intensive method of dispute resolution, google would likely (and often does) fight it.
But court cases are just really expensive and time consuming. The average civil litigation takes 5-7 years to resolve.
