While it is never addressed in-universe, MCU director Joe Russo confirmed Thanos was around 1000 years old in 2012's The Avengers when he first appeared in the MCU. Based on that, Thanos is younger than Thor and Loki, which, if revealed in one of the past two Avengers films, might have proven itself to be distracting.
He's roughly over 1000 years, somewhere around 2000 or 5000. But in human years he's as old as the actor that he's played by-Josh Brolin which is 51 years old. This would make sense because Gamora is in her late teens and early adulthood and when Thanos captured her for the first time she was a child.
Advanced Longevity: It has been confirmed that Thanos is an extremely long-lived being and was roughly over 1000 years of age by the events of the Infinity Wars and the Battle of Earth. Even during the 2010s, he had displayed no signs of aging or any decline in his physical powers.
He has feats such as defeating Surtur (who can destroy a planet), conquering the 9 realms, and sealing away Hela. While there isn't much doubt that Thanos could beat Odin if he had all 6 stones, Odin could probably defeat Thanos if he only had one or two of them.
In Thor: The Dark World, Odin reminds Loki that Asgardians are not immortal and have lifespans as humans do, to which Loki responds: "Give or take 5,000 years." From this, it can be calculated that Asgardians live to about 5,100, with Loki being around 1050 years old in the MCU.
While Steve Rogers gets the title of the oldest human avenger, Thor Odinson, as the mighty Asgardian God of Thunder, has a much longer life span than any of his Earth-born friends.
While it is never addressed in-universe, MCU director Joe Russo confirmed Thanos was around 1000 years old in 2012's The Avengers when he first appeared in the MCU.
Based on what can be deduced about his species' aging, it is possible that Groot is around 14-18 years old in The Guardians of the Galaxy Holiday Special.
He was finally defeated after his granddaughter, Nebula, stole the Infinity Gauntlet from him, robbing him of his power—but now the power was in her hands, and she was no better than her grandfather. Adam Warlock recruited Thanos to help defeat her, which he did after realizing that he was unworthy to wield such power.
In the aftermath of his failure, Thor had a new-found determination to kill Thanos and undo the Snap. The remaining Avengers converged on the Titan's home to find him wounded and the Infinity Stones destroyed. Seeking vengeance, Thor raised Stormbreaker and, with one fell swoop, beheaded Thanos.
All of these factors lead to Thanos being easily able to lift 100 tons, putting him in the same category as Thor, Hulk, and powerful celestial beings. Thanos is also skilled with hand-to-hand combat and was trained in the Art of War while on Titan.
Thanos is a mutant member of the race of superhumans known as the Titanian Eternals. The character possesses abilities common to the Eternals, but amplified to a higher degree through a combination of his mutant–Eternal heritage, bionic amplification, mysticism, and power bestowed by the abstract entity, Death.
Thanos is a boy's name of Greek origin and flows from the name Athanasios, meaning “immortal.” Thanos is known in Greek mythology as a shortened name for the Greek god Thanatos.
As Cates has pointed out on Twitter, it's important to point out that “Thanos” is still Thanos' real name, and that “Dione” was what his mother originally wanted to call him before he was born.
Thanos was first introduced by Marvel in The Invincible Iron Man #55 in February, 1973. Thanos has a height of 8'3” (2.52 m) in the Marvel Cinematic Universe and 6'7” (2.01 m) in the comics.
Thor was born approximately 1500 years ago to Odin, the King of Asgard, and Frigga. He was raised on Asgard as the crown prince, alongside his brother, Loki.
Thor. Blessed with the powers and long life of a god, Thor was undoubtedly the most powerful member of the initial version of the Avengers. Not only can he fly and transport between worlds, but he was also the only one capable of taking down Thanos, even after he had assembled all six Infinity Stones.
As Bruce Banner, the character is about 5 ft 9 in (1.75 m) tall and weighs 128 lbs (58.05 kg), but when transformed into the Hulk, the character stands between 7 and 8 ft (2.13 - 2.43 m) tall and weighs between 1,040 and 1,400 lbs (471.73 - 635.02 kg).
In Thor: The Dark World, Odin makes it clear that they “are born, they live, they die, just as humans do”, to which Loki replies that “give or take, 5000 years” – so taking that into account, Odin was at least 5000 years old when he died in Ragnarok.
Chronologically Cap is 66 (the years he was frozen) add his “true” age of 39, giving us 105. After the jump to 1945, he presumably lived until 2023, which gives him an added 78 years to both of his ages. So overall, biologically Steve is 117, chronologically he's 183.
High says that Loki's alternative name is Lopt, that he is the son of the male jötunn Fárbauti, his mother is "Laufey or Nál", and his brothers are Helblindi and Býleistr.
