While “NCIS” actually started as a spin-off for a forgotten show, which is where Mark Harmon made his debut as Leroy Jethro Gibbs, Gibbs is best known as the main face of “NCIS,” beginning with the show’s debut in 2003. Harmon played Gibbs for a whopping 19 seasons of “NCIS” before stepping away. He’s yet to return to the main series despite the show having continued on for three additional seasons and counting. Gibbs aged in real time between 2003 and Harmon leaving the show in 2021, being 47 at the start of the series and in his late-60s by the end of his stint. 

In 2024, an “NCIS” prequel series called “NCIS: Origins” debuted on CBS which follows Gibbs when he was just starting out as an NCIS agent and was still fresh off the deaths of his wife and daughter. It’s set in 1991, 12 years before the events of “NCIS” started. Austin Stowell took over the role as a 37-year-old Gibbs — though Harmon appeared as a current-day Gibbs in the premiere, which, being 2024, would make him about 72 years old here.

Frequent hints at Gibbs’ age — including when he was born, when he lost his wife and daughter, when he graduated from the academy, and other milestones in his life — made it possible to at least take an educated guess about his age during the first 14 seasons of “NCIS.” However, in the Season 15 episode “House Divided,” audiences are shown a very clear shot of his ID, including his birthdate. 

Given that Gibbs was born on November 21, 1954, and that “NCIS” began in 2003 and seemed to take place in the modern day, that would’ve made him 49 years old at the start of the series and approximately 67 when his arc on the show came to a close in 2021. As for Mark Harmon himself, his age is pretty close to that of Gibbs: Harmon’s birthday is September 2, 1951, making him three years older than Gibbs. Harmon was 51 when “NCIS” debuted and 72 when his final episode aired — although he almost quit “NCIS” a lot earlier. 

The prequel series “NCIS: Origins” begins in the early-90s when a 37-year-old Gibbs is trying to balance the start of his time as an NCIS agent with the struggle of having just lost his family to murder. It only has one season under its belt so far, and we don’t yet know how many years the show will last nor how much of Gibbs’ life it will cover, so it remains to be seen how old he is going to be when “Origins” inevitably wraps up. But it’s worth noting that current-day Gibbs (Harmon) is the narrator of the prequel series, and given that the “NCIS” universe has continued to advance in real time, that would make this version of Gibbs around 72 during the first season of “Origins.”

If you’re wondering why young Gibbs from “NCIS: Origins” looks so familiar, that’s because Austin Stowell has a number of film and television credits under his belt. His most notable credits are the movies “Whiplash” and “Bridge of Spies,” and he was also in the Billie Jean King biopic “Battle of the Sexes,” playing her husband, Larry. Interestingly, Stowell also previously played a small role in an episode of “NCIS: Los Angeles,” but it wasn’t as a young Gibbs — he appeared as Marine Corporal Andrew Peterson in the 2010 episode “Special Delivery.”