The chapters of the GF novel which Puzo used for the basis of his GF IV script, which would presumably be most of the "flashback" storyline, cover the time period in the mid-to-late 1930s when the Corleone family rises from being just neighborhood gangsters, to a power on the national organized crime scene.

Vito would be about 40. Hyman Roth, Tessio, Clemenza and Pentangeli, about the same age maybe up to about 50. (In fact, since we know Roth is 67 in 1958, we can say he'd be 45 in 1936.)

Sonny and Tom and Fredo, early twenties. Michael, late teens to maybe 20. Connie, mid-teens.