There are a lot of fan theories out there about the age of Big Boss in Metal Gear Solid 5. The most popular theory is that he is actually quite old, around 70 or 80 years old. This would explain why he looks so different in MGS5 than he did in previous games.
It also would make sense given his backstory and the events that have transpired since Metal Gear Solid 4.
Big Boss Biography – Metal Gear Solid V: The Phantom Pain
Metal Gear Solid 5: The Phantom Pain is set nine years after the events of Metal Gear Solid 4: Guns of the Patriots. This would make Big Boss approximately 62 years old during the events of The Phantom Pain. However, due to his extensive cybernetic surgery and time spent in a coma, he appears to be much younger than his actual age.
Despite his age, Big Boss is still an incredibly skilled soldier and strategist. He’s able to hold his own against enemies that are half his age, and he always seems to be one step ahead of everyone else. It’s clear that even at 62 years old, Big Boss is still a force to be reckoned with.

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How Old was Big Boss in Metal Gear?
Big Boss was born in 1935. He was 57 years old when he died in Metal Gear Solid 3: Snake Eater.
How Old is Solid Snake in Mgs5?
Metal Gear Solid 5: The Phantom Pain is set nine years after the events of Metal Gear Solid V: Ground Zeroes. This makes Solid Snake approximately 49 years old during the events of The Phantom Pain.
During his time in Afghanistan, Big Boss went by the codename “Naked Snake” while he was working for the CIA.
After the destruction of Mother Base, Big Boss sets out to create a new private military company called Outer Heaven. It’s at this point that he takes on the codename “Venom Snake”.
Many fans have speculated that Venom Snake is actually a clone of Big Boss, created using DNA from the original.
However, this has never been confirmed by Konami or Hideo Kojima.
How Old is Big Boss During Snake Eater?
Big Boss is 47 years old during the events of Snake Eater. He was born in 1935, making him 29 years old at the time of the Cuban Revolution, and 38 during the events of Operation Snake Eater.
How Old is Big Boss in Peacewalker?
In the Metal Gear Solid games, Big Boss is referred to as a man in his late twenties or early thirties. However, according to the Peacewalker timeline, he was born in 1935. This would make him around 44 years old during the events of Peacewalker.
Conclusion
In Metal Gear Solid 5, Big Boss is around 70 years old. However, he’s been in a coma for the past nine years, so he’s technically still in his 60s.