Thanos has been the pinnacle villain of the MCU's Infinity Saga, captivating audiences for years; however, his history seems to span farther than a few decades. With fans wondering how old the Mad Titan is in the MCU, Avengers: Endgame director Joe Russo has confirmed his age, and it’s a grand number. Russo claims Thanos is around 1,000 years old, calling him to be "the Genghis Khan of the universe." The filmmakers could have chosen any reasonable age for the villain, but having Thanos be around 1,000 years old respects the larger canon of the comics, which, while vague, keeps him around this age.
In the comics, the real age of Thanos is murkier and less explicit. Thanos sprung from the creative mind of writer and artist, Jim Starlin, in the early 70s, but the first complication to his possible age appeared in print just before his appearance with the character Moondragon, who was taken by Thanos' father and grew up on Titan. Her ever-evolving origin story would fuel much of Marvel's difficulties in developing Thanos' history, but it gave a vague range for Thanos' age, establishing him as Moondragon's elder later on.
With Thanos being around 1,000 years old in the MCU, he is far from its oldest character, at least in terms of the celestial heroes and villains. Looking beyond his children and several Guardians of the Galaxy, Thanos is one of the younger characters of Marvel's intergalactic being; however, what Thanos lacks in age, he makes up for in size and strength, but youth is definitely on his side in a galactic sense.
For instance, all of the predominant Asgardians are older than the Mad Titan, and Loki is the closest in age at 1,054 years old. Thor's age is less defined, but he is between 1,200 to 1,500 years old in the MCU; meanwhile, Hela, who helped Odin conquer the Nine Realms and spent a millennia in prison, is a few thousand years old. Similarly Valkyrie, who faced Hela before her imprisonment, also spent time incarcerated, putting her around the same age as the Goddess of Death. Of course the oldest of the predominant Asgardians is Odin at 5,000 years old, so all of them are elders to Thanos.
Beyond the Asgardians, there are also the Elders, including the Grandmaster and the Collector. As seen in Thor: Ragnarok, the Grandmaster is, "the first lost and first found" on Sakaar, and he later confirms that anywhere else in the galaxy, he would be millions of years old. It's also confirmed that the Elders of the MCU first appeared shortly after the universe began, making them significantly older than Thanos.
For Thanos, especially as such an important and genocidal antagonist, little details like his age don't add much to his mythos, but it does put things in perspective when compared to the older characters of the MCU. As commonly implied, with age comes experience and wisdom, and while Thanos has more experience and wisdom than most of the human heroes, he is still young compared to other characters. If Thanos were older in the MCU, as is implied in the comics, it could perhaps give an even more complex interpretation of his motivation to wipe out half of life in the universe in order to save it from itself.
His plan is dastardly, but for Thanos, he is doing something right even if it is the worst possible approach. The younger Thanos is compared to other prominent figures in the MCU, the more his motivation to wipe out half of life seems like the misguided aggressions of youthful ambition rather than the misguided wisdom that sometimes comes with age.