In black beauty how old was little joe when he became john manley's assistant

The correct answer is: Little Joe was 15 years old when he became John Manley's assistant. He is called "Little Joe" not because he is young but because of his size.

At first, Joe Green is the steady kid for Squire Gordon and Lady Anne at Birtwick Park; he replaces James, who in Chapter 16, after the Squire and his better half have chosen to visit companions and remain at an inn, salvages Beauty and Ginger from eating after an indiscreet man has entered the animal dwellingplace, smoking a pipe. In Chapter 17 the fifteen-year-old Joe starts work under the tutelage of James.