The Picture Book...or otherwise known as a PB usually targets children ages 2-4 and 4-7 years old and is under 500 words in length. A PB for children ages 7-9 could be up to 1000 words in length.
The Chapter Book (can be a series) can target children ages 6–9 ( Young readers: 800 to 1200 words) or age: 7–10 years can range from 5000 to 25,000 words.
Middle-grade fiction (or MG) is defined as fiction featuring pre-teen characters in situations of interest to 8-12 year olds. Middle Grade novels are usually between 30,000 and 50,000 words long.
Young Adult or YA fiction features teen protagonists in situations of interest to teen readers (12 or 13years and older). YA novels are usually between 50,000 and 90,000 words long.