Hey Lostpedians, just saw something interesting on Lostpedia, on the Main Page, first off, and then on the Autos page. On the main page it states that "None of the cars used during the scenes where Ben was born were made after 1963", and on the Automobiles page, it's a 1960-1974 Volkswagen Karmann Ghia convertible.
So have we have pinned down Ben's exact birthday?
December 22, 1960-1962, making him 38-40 on-island. Seems plausible to me.
EDIT: I've recently eliminated the 1963 model. The windows in the episode are silver, while on the 1963 are black. Still searching for the correct model. If there are any better shots of the car, please let me know.