Don't you remember the scene outside their apartment after Vito kills Fanucci and all of their kids are gathered around including Sonny who would have been about ten or so?
Michael is a newborn in that scene. Is there really 10 years between Sonny and Michael?? Sonny might be closer to 10 when they go to Italy (when he gives the older woman the Statue of Liberty), but that's several years later.
In the movie, Sonny was about 4 or 5 when Fanucci was killed.
