I assumed age appropriate was regarding content not difficulty. It is still a weird thing to emphasize though.
They undoubtedly wouldn’t approve of the content of some of the books I was reading back then either. I had already learned the extremely broad strokes around sex and reproduction by the first grade. My parents have a farm with livestock. I was also reading computer manuals learning how to be a greyhat, before the term even existed.
It always strikes me regarding the mental gymnastics people engage in regarding consumption of entertainment. Violent video games*, even if it’s cartoon violence, tv and movies are everywhere. But people clutch their pearls if it’s in a book format. The world is ending if it’s sexual. Hell, Utah just banned Judy Blume books.
*I’m not condemning video games, study after study has proven that violence in games doesn’t lead to violent behavior, just that we find violence in games acceptable vs people losing their shit over a girl getting her period in a book for YA’s.