Completely false. Lots of people in their 20s have knee and back problems. Torn ACLs are common in athletes and people who participate in sports. I had a coworker a few years ago who had his ACL replaced; he was in his mid-to-late 20s.
Your statement is blatantly false. It says, literally, you only get knee and back problems when you're old. This is wrong, and stupid.
Why are you arguing this?