Why did Solid Snake age so fast?
He faked his own death in the Manhattan Tanker Incident, though he re-emerged years later to assist Raiden in the Big Shell Incident. During this time, his health declined as his body entered a state of accelerated aging due to intentional genetic changes made during the cloning process.
forty-two years old
As a result, Snake has the physical appearance of an elderly man, despite being only forty-two years old.
Solid Snake has a mild form of Asperger's Syndrome or another high-functioning autism spectrum condition.
FOXDIE virus
The FOXDIE virus that was injected into Solid Snake's body in 2005 (left), alongside the mutated strain that was present in his body as of 2014 (right). FOXDIE was an engineered retrovirus developed by the DIA for the Pentagon.
In perfect conditions, adult snakes live anywhere from 20 to 30 years, with natural predators and the encroachment of humans severely limiting the number of years most snakes live.