If someone can help fill in the gaps below, I'd be eternally grateful!
I would tweak it a bit, as follows. Some of the categories overlap, of course, and a lot of kids read up (or down).
Age 0-3 board book (few words, if any)
Age 2 - 5 picture book (more words but story is highly dependent on pictures. Usually under 1000 words.)
Age 4 - 8 picture story book (longer, more complex story but picture on every page.)
Age 5 - 8 early reader (still with pictures on every page. Word count variable; most publishers have their own policies re: word count
Age 7 - 10 chapter book (still with pictures but not every page. Usually 5000 - 10,000 words. Occasionally up to 12K for higher end. Often sold in series. Again, publishers have different policies on preferred chapter book length.)
Age 8 - 12 MG (few pictures, 30, 000 to 50,000 words, sometimes longer)
Age 10 - 14 Upper MG or "tween" (in between MG and YA, often have a strong commercial hook)
Age 12 - 16 YA (lots and lots of words, no pictures)