While I agree that is what is supposed to happen, it rarely does. My father worked for a company for 30 years and was fired for age discrimination (his reviews were always good, well liked at the company). He basically wrote everything there was for training new employees at the company as well and was the head of their training department.
However, they brought in a new HR director and CEO that wanted to "shake things up" and the HR guy had a buddy from his former company that wanted to come in and work with him and happened to do a similar job that my dad did. You can probably guess what happened next, my father was fired without much of a reason. Those in executive positions that could have stood up for him were either in the minority or retired, so he had little help to back him up.
My family pushed him to sue, but he felt pressured into taking their severance package because he had to support my brothers and my mother and was worried about his prospects of getting a job in his 50s at another company (despite having a Bachelor's and Master's degree from well-known schools). He ended up taking the severance and waived his right to sue (at least according to the agreement he signed). He was also afraid that suing would blacklist him from employment elsewhere and it was hard for him to bring himself to sue the business he once loved. Coming from the age when many were loyal to their companies, even if the company was not loyal to them, he just couldn't do it.
Since then, he's applied at numerous employers for similar jobs to his own as well as other ones he could segue into. However, no one wants to hire him basically because of his age despite no intentions of retiring anytime soon. Now, he works a low wage job he could get without a degree from a university and it just kills me to see him like that. My only wish is to eventually grow my own business enough I can hire him myself so he can do something more respectable. He's not a huge tech guy like me so getting him into software development with me is a bit more than possible, but he's a smart guy that would fit into many other business and training related roles. However, those roles are something I can't pay for when I'm either working for clients, my own projects or with a friend right now.
Although age discrimination is against the law in the US, it's much more complex than it appears for most cases and suing is a difficult question that could hurt as much as it could help. I've always been reluctant to talk about my father's ordeal, but I felt I had to get it off my chest at some point.
True, most discrimination suits are not cut-and-dried, but a document that says "Let's fire the oldest worker that makes less than six-figures." is the smoking gun most cases salivate for.
And, if you're in the U.S. and the employee is over 40 years old, you automatically set yourself up for an age discrimination suit.