Re, Gibbs' age: we know Gibbs has to be a little younger because NCIS agents have to retire from the field at age 55. (Ironically, Mark turned 55 the year Gibbs temporarily retired on Hiatus, last year)
He wasn't considerably older than 37 in fall of 2003 (his words). The casting description apparently wanted him in his early forties. He was too young for Vietnam. And if the producers were using JAG's run as a guideline, they'd want to keep him young enough to be age eligible for a ten year run, which would assume he's probably not born before 1958.
I go into a bit more detail on how I guessed his approximate birthdate in the Gibbs bio I have in the fanfic thread.
"You don't sound very optimistic. So all these people just die?"