Actually Kerry was 19 at the time—he was born 12/11/1943, so he was just shy of 20. Curiously, Kerry was almost precisely the same age that Gavrilo Princip was when he shot the Archduke. Does that make Kerry the second shooter?
Definitely the second shooter. This picture shows a young man who looks like Kerry peering over the picket fence on the grassy knoll. can there be any doubt?