Why does snake age so much in Metal Gear Solid

The reason why Solid Snake ages so dramatically from MGS1&2 to MGS4 is because of this;

Solid Snake is a genetic clone of Legendary soldier Big Boss (aka Naked Snake or Jack). He was born from the Les Infants Terribles project conducted by Zero and Dr. Clark (Big Bosses former allies) in 1972 along with his brother Liquid Snake. Solid Snake and Liquid Snake were created as an insurance policy to make sure the Patriots (Zero's Organization) would never lose their Icon (Big Boss). But because of the risk posed to the group if the Snakes were taken from the patriots hands Solid Snake and Liquid Snake were genetically altered to age twice as fast as a normal and are unable to procreate. An insurance policy inside of an insurance policy. This would limit the amount of the Snake's use an enemy organization would have against the patriots. Plain and simple; Solid Snake was engineered to have that as his natural lifespan.