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'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.
Directors Joe and Anthony Russo and writers Christopher Markus and Stephen McFeely provide the commentary, and they revealed that Thanos chose to wait because he had no idea about the location of the Soul Stone (via io9).
Thanos being revealed in-universe to be around 1000 years would have repeated a long-running villain cliché idea that is based on villains being described as an ancient evil or timeless threat.
Yes, as an Eternal, Thanos is immortal by human standards. Immortal meaning can not die by disease, or old age, as in natural causes. Thanos is NOT immortal but extremely long lived (much like Asgardians, he does age).
Franklin Richards is the immortal hero of the multiverse
In the Marvel comics, Franklin Richards has the power to make thoughts real. But in one of the alternate universes, Richards of Earth 12665 has become immortal. He is the adult version of the character and managed to gain powers to defeat two celestials.
If Thanos couldn't have Death, no one could; it was the one thing that Thanos could not and would not abide. When he saw that Deadpool was putting the moves on his crush, Thanos decided to intervene in a decidedly drastic way: he cursed Wade Wilson to eternal life.
Thor. 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.
Can Thor beat Thanos? In the MCU he certainly would have, if only he had aimed for the head. The Asgardian Son of Odin was extremely close to ending Thanos at the end of Avengers: Infinity War. After gaining the powerful weapon Stormbreaker, Thor becomes the wielder of the most powerful weapon in the universe.
Thanos didn't lift Mjolnir, Captain America did
With Mjolnir's worthiness enchantment resulted in Thor being able to lift the hammer only if he is considered to be worthy, but likewise, the enchantment is not discriminatory in nature, and anyone who is deemed worthy is able to lift the hammer.
As per the user, the director of Avengers confirmed that Thanos was most scared of Iron Man. “Not because of his strength, not because of his power, not because of his power, but because of his suit,” he added.
After escaping to the Garden and destroying the Stones, he is eventually killed by Thor. Five years later, an alternate version of Thanos from 2014 time travels to 2023 to battle the Avengers once again, but is killed by Tony Stark.
Let me please remind you, that thanos was more scared of Wanda then of any other avenger.
Thor is Immortal, Not Invincible
In Avengers: Infinity War, Thor revealed that he was over 1500 years old. It's clear that age doesn't affect him the way it does normal mortals, as with most immortals, he is aging at a slower rate. While Thor will eventually die, it will take thousands of years for it to happen.
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.
Meanwhile, Groot was around four years old during the events of the Avengers: Infinity War and Endgame. Then, in The Holiday Special and Vol. 3, he is around five to six years old. If you count the time he was snapped as lifespan, in that case, he is around 11 years old in Guardians of the Galaxy Vol.
Additionally, Wanda demonstrated just how powerful she was in Infinity War, when she was able to repel Thanos while simultaneously destroying Vision's Mind Stone. So it stands to reason that Wanda would have been able to single-handedly whoop Thanos into oblivion, had he not called in cover fire from above.
Superman has beaten everything the Multiverse has thrown at him and always come back for more. He's faced Thanos's type many times and always won. Superman is vastly more powerful and skilled than Thanos. It wouldn't take him long to realize how dangerous the Mad Titan was and once he did, he'd take the kid gloves off.
That makes Vision the youngest Avenger.
Jeanne Calment, a French woman, achieved an incredible feat of living to age 122, thus earning the honor of being the world's oldest person on record.
In the original Infinity Gauntlet, Thanos uses his omnipotent power to create a new wife, Terraxia, revealing the truth of his motivations.
Hela, the Asgardian Goddess of Death, rules two of the nine realms: Hel, land of the dead, and Niffleheim, land of eternal ice. With a simple touch, she can cause gods to perish, but Hela's purpose is to receive Asgardian souls, as well as their followers.
Thanos' beloved, the hooded embodiment of the end, should make an appearance before the Thanos saga is over. Something about that cloaked figure and her skull visage standing next to Thanos is so iconic. Death's inclusion would make audiences understand (and fear) Thanos as he tries to burn a galaxy to win her love."