A lot of guys in high school didn't have one until they were told in junior year to get one because you needed it for all the college applications.
For more recent children, since SSN of the kid is now required on income tax return when parents claim them as dependents, it’s probable that a large percent of newborns get their SSN at or soon after birth, which would increase the “hit rate” of an algorithm based on birth date and state at time of birth.