Well, some of the novel bleeds over. There, Sonny follows Vito across the roofs when he's about to kill Fanucci. I can't see a kid much younger than about 10 doing that.
You're mixing apples and oranges.
Sonny never saw Vito kill Fanucci in the movie. Yes, in the novel, Sonny was born in 1910 (he's described as being 35 years old at Connie's wedding in 1945). In the movie, Sonny was born in 1916, so there's six years difference between Sonny's age in the book and the movie.
