You're misreading the sentence, he's not saying people should be fired for being old. He's saying at Google you can not fire someone for the "plain old reason" of "they're not doing a good job".
Having worked at Google, it is extremely difficult to fire a low performer. It takes about 6-12 months, many visits with HR, and a ton of documentation. So there is a common practice of managers trying to offload their low performers onto other teams, as mentioned in the post.
Having worked at Google, it is extremely difficult to fire a low performer. It takes about 6-12 months, many visits with HR, and a ton of documentation. So there is a common practice of managers trying to offload their low performers onto other teams, as mentioned in the post.