Gibbs replies to the NCIS Director that he wasn't old enough for service in Vietnam. Gibbs cites only two combat tours, Panama and Desert Storm. From this we can assume that Gibbs is younger than the actor that portrays him. Not only is Mark Harmon old enough to have served in Vietnam, being born in 1951 he was still subject to the draft under the 1970 draft lottery which pooled its draftee selections from young men born in the year 1951. While the path to military service and deployment to Vietnam was different and less than certain in the period 1971-73, it would have been conceivable for Gibbs (as portrayed by Harmon) to have been written as a Vietnam veteran if they had chosen to do so after Harmon had been cast in the role. The NCIS director concludes that part of their conversation remarking that he thought Gibbs was older.