I'm sure, like most rules, it's mostly just a guideline to handle the common case. People have all kinds of circumstances in life, so drawing the line on an age is often something that can be explained through. As a totally unrelated example, I have a Jewish friend that just turned 27 and got an exception made for him for Birthright Israel even though the cutoff is 26.
I talked to a 20 year old and told him what you said about rules not really applying. He's exceptionally well qualified for the fellowship and two degrees of connection away from Thiel/Jim O'Neil and told me that he asked, and was told decisively that 20 year olds are out.
That's really, tremendously depressing. There's a huge list of reasons to follow rules exactly to the letter. Enforcing this one, which should be entirely about potential and not at all about an arbitrary date of birth, is inane.
Well it's called 20 under 20. Slippery slope, etc.