MGS4 is set in 2014 which is 5 years after the Tanker chapter in MGS2.
But look at "solid snake" in MGS4 and he has at least aged 30 years since MGS2. -White hair, wrinkles, etc.
Not to mention technology also seems to have progressed a great deal in 5 years.

Also Otacon, Meryl, Vamp all look near the same age as they did in their respective debut games so then it makes me think that Solid Snake isn't Solid snake at all, but is actually Big Boss -Hence why he was standing at the graveyard reminiscent of MGS3's ending and why he looks much older. Sure, he died at Zanzibar or so we know but knowing Kojima he'll throw a plot twist in to explain it all.

I thought i read something somewhere about how snake got infected or something with a disease that made him grow old fast?

Wha if thats snake's father!!!

Solid Snake is a clone.. his DNA ages fast.. so he gets old fast...

 Big Boss IS Snakes father -Or at least he was cloned from Big Boss.

And infected with a disease, I didn't hear that? I know FOXDIE will be unleashed on the battlefield but others including Meryl have been infected by it and they don't look old.

In one of the trailers raiden says "My turn to protect you snake" like they've encountered each other before.

isn't cloning humans illegal.... ???

ok I go out.... lol :)

 Yeah, I also figured this was the reason. In laymens' terms, basically a clone's DNA is the same age as the DNA of the person who got cloned. This is the reason why scientists believe that original cloned sheep died so quickly. 

Er... it's exactly what ssj12 said. It's because when 'them' took the samples from Big Boss, he was in his 50's. He was old material.

That's why Solidus looked so old on MGS2, but in his case he aged faster than Liquid and Solid. Also Solid looked like he aged ten years in the two year lapsus between the tanker incident and the plant incident in the same game.

OHHH thats how.

Sorry guys didnt know what had caused him, i just knew that something was wrong with him and he was growing old