I believe Caddy is the oldest, followed by Quentin, Jason and Benji. In 1910, Quentin is a freshman at Harvard. That would make him six in 1898, and probably make Caddy around eight. I'd say Jason is around four or five in 1898, making him 34-35 in 1928 (Benjy's 33rd birthday). It's obvious that the four of them are fairly close in age, all born between 1889-1895.