I'm not a Microsoft Employee, but know a few former and current ones. "Career maximum" is not “age-related discrimination".
Career Maximum means your management believes you are at the highest level you are capable of handling. If you are a great coder, but unable to handle the running meetings to bring together multiple groups to agree on common functionality for a feature, and you are considered unlikely to develop this skill... Well, you can't really expect a promotion to the level where that would be your job, can you?
Now, the actual implementation of this isn't perfect- How long do you wait before you decide someone is unlikely to develop a skill? How do you decide that someone can't, say, run a meeting? You even have a bit of a skew, in that an older employee is likely to have been promoted a few times, so they are more likely to have reached whatever maximum they might have. And what do you do with an employee who has reached their maximum? Keep them happy, well fed, and in the same level of a job, or get rid of them and hire a new person who might top out higher?
Of course, individual managers might use this as a tool to discriminate against older employees, or even blue eyed employees with six fingers on their left hand, but that’s true of any tool with any sort of subjective metric.