He was raised in Westchester County and went to Philips Exeter, then Harvard.
Many others have as well, and have not done what he did. But that still doesn't qualify as "starting from zero" or "nothing to show."
How many people do you know who started off at an age of 19-20 and were a tech celebrity in 2-3 years? Well I do not know of many hence they are and will be an outlier for me.
A lot of things including luck and timing beside the usual clicked for both Mark and Bill and hence they are where they are today!
The two arguable factors are luck and outside capital. Are you sure luck played such a great factor in his case ? and was his decision to take outside funding at that stage unnecessary?