“My parents would frisk me before family events. Before weddings, funerals, bar mitzvahs, and what have you. Because if they didn’t, then the book would be hidden inside some pocket or other and as soon as whatever it was got under way I’d be found in a corner. That was who I was…that was what I did. I was the kid with the book.” —Neil Gaiman (via booksandnerds)
Voldemort rose to power in 1997. He destroyed all records of muggle borns. Therefore, all muggle borns that would have been younger than 11 at the time of his takeover would never have gotten their letters because he's an evil wizard and destroyed our records. The Ministry is still in the process of finding all muggle borns.