… either theirs or their children’s:
Essentially: by the age of about ten, you’re either going to have got it or not. Sure, you can train yourself alter, but some folks just get it, some folks just don’t. It doesn’t mean you’re clever, dumb, a good or bad writer either way.
But if you’re going to be a copy editor, you’d better damn well be able to do it either on your own or with the help of a handy reference. And the automated spell checker is NOT your friend.