I have kids and it seems like fine advice. You find books they'd be interested in by knowing about their hobbies, interests, and personality, and finding books that match that in some manner, and then pitching it to them, regardless of whether it is age appropriate or not.

Of course it is hard to do - at least, harder to do than saying "I have a 5 year old, and this book says it is for 5 year olds, so I will have them read it".