Thanos is the ultimate villain in both the comics and the movies, but exactly how long has he been wreaking havoc on the Marvel universe? While Avengers: Endgame co-director Joe Russo stated that Thanos is 1,000 years old in the MCU, the Mad Titan’s age in the comics is much more difficult to determine.
Thanos made his debut in Iron Man #55 by Jim Starlin back in 1973. In that issue, Iron Man is given an abbreviated version of Thanos’s backstory by Drax the Destroyer, who at the time had nothing to do with the Guardians of the Galaxy. In fact, he was actually created by Thanos’s grandfather, Kronos, for the sole purpose of killing Thanos. In order to understand why - as well as get closer to figuring out Thanos’s true age - let’s start at the beginning.
While variations of Thanos’s backstory have been told across many different stories in the Marvel canon, they all more or less feature the same major plot points. The Thanos Rising mini-series from 2013 by Jason Aaron gives the most detailed account of the villain's origins. Born on the moon of Saturn called Titan, Thanos is the son of the world’s foremost scientist and leader, A’lars, AKA The Mentor, and a woman named Sui-San. They belong to a race called the Eternals, beings with many extraordinary abilities, one of which is immortality. However, while most of the Eternals look like stereotypically beautiful humans, Thanos was born with a mutation that caused him to have an unsightly, purple visage. Because of this, Thanos’s mother tried to kill him right after his birth, but The Mentor stopped her. They later have another son, Eros, who grows up to become Starfox.
Growing up, Thanos had an increasing fascination with death, which culminated when he killed his own mother at the age of ten. The Thanos Rising series states that Thanos left Titan that same day, but according to The Mentor’s account of the incident in Silver Surfer #37, Thanos did not leave until eight years later after committing another murder. Regardless, Thanos left his home between the ages of 10 and 18 and started working for intergalactic pirates. He eventually usurped the captain and took over the ship, slowly amassing more forces until his crew became an army. Thanos then returned to Titan after more than eight decades away, making him around 100 years old, and decimated the planet, leaving only his father alive - a mistake he would later come to regret.
After Thanos leaves the destroyed Titan, A’lars contacts his father, Kronos, and asks him to kill his son. Enter Drax the Destroyer, who spends an unspecified amount of time searching for Thanos and eventually finds him on a distant planet far from their solar system. Thanos manages to defeat Drax and takes him captive, but Drax uses his telepathy (yes, he can do that in the comics) to call Iron Man for aid, marking Thanos’s first encounter with an Avenger. Based on all this, it would seem that Thanos is a little over 100 years old, but there are some complications. Starfox, Thanos’s younger brother, is said to be over 1,000 years old, which would mean that Thanos is even older than that. Exactly how much older is unclear, but it’s probably only a few years difference since the two grew up together, and Eternals seem to initially age at the same rate as humans before tapering off into immortality. Even so, other characters like Mephisto have also alluded to Thanos being around for centuries.
The only way this makes sense with the timeline established above is if Drax spends the better part of a millennia searching for Thanos, which seems unlikely, but then again, this is a guy who thought he could turn invisible by standing still, so who knows. Regardless, there is really no definitive answer to exactly how old Thanos is in the comics - we just know he’s been around for a very long time, and will likely continue to terrorize the Marvel universe for years to come.