I’m still concerned about some dates in the novel. According to the novel Vito Andolini was born in 1891 (the movie says 1887, part II says 1892) and stayed in Italy untill he was twelve years old, so untill 1903 (part II says he leaves for America in 1901). The novel says 18-year old Vito marries 16-year old Carmella, so in 1909, while the movie/DVD says it’s in 1914. After their first year of marriage, to the book, Sonny is born. This must mean 1910 then, which is accompanied by the facts of the sequences of Tom Hagen’s background and Connie’s wedding, where it is said both Tom and Sonny are 35 years old at that time (which is soon after WWII). According to the DVD/movie trilogy though Sonny is born 1916, and Tom in 1910. According to the Maranzano sequence in the book, Sonny is born in 1917 (in chapter XIV it says Santino was a 16 year old young men on New Year’s Eve 1933, which would mean he’d get seventeen the year after. This would mean that Tom is either born in 1916 or ’17 as well). Also it says that Tom Hagen at that point has been living with the Corleone’s for three years, so from 1930, when he is 13 years old, and not eleven years as stated before. If he really ís born in 1910, then he had come to the Family aged twenty years.

Also, the Maranzano sequence as a whole is dated wrong, but this I remembered from the deaths of the popes in part III as well, which are inaccurate to the real popes’ deaths. In the novel, Maranzano gets killed in a restaurant in 1933. In the real mafia-history, Maranzano’s nemesis Joe the Boss Masseria was killed this way in 1931, after being betrayed by Lucky Luciano in the restaurant. After this Maranzano organizes a big meeting where he introduces the La Cosa Nostra-structure. This structure is the beginning of the Five Family’s and the Family-chain of Don-Consiglieri-Caporegime-Soldier-Buttonman. Manzano becomes the first and only ‘capo di tutti capi’. In the book, Genco Abbandando already is Vito’s consiglieri in the mid-twenties. The real Maranzano get’s killed anyway, but that was in 1931 and in another way (hitmen dressed up as IRS-agents surprise him in his office).

So we have confusing information about birthyears of Sonny, Vito and Hagen, and misplacements of some historical facts.

Everything about Sonny and Tom’s ages would be straightened out if we’d assume the books early facts of them being 35 at Connie’s wedding are wrong, and accept they are born in 1916/17, I would be a lot happier. Still, I don’t know why the movie-Vito had to be born in another year and leave for New York another year than the book-Vito. And, not that important though, why his marriage-dates are different is a surprise to me too.

I got the 1891 myself, but not in the same way you described it, but from a little math from the book, which I can't place anymore. But your explanation is very cleaver.