So, Alexander Selkirk was 28 years old when he had Captain drop him off because the ship they were on wasn't seaworthy

 So, this means he lived from age 28 to age 32 before he was rescued. The ages he was likely was why he survived this alone. Someone either much older or younger than around this age likely would not have survived all this being alone like this. However, the peak physical state is around 32 to 33 years of age for a human male.

The other thing would be that without enough survival experience (mostly through one's 20s) one often would not survive this either. Also, he was a Naval Officer and so likely well trained and somewhat educated in order to be a Naval Officer to begin with.

Also, the things he went through on the island also might be one of the reasons that he later died at age 45 Somewhere in Africa after being rescued.