olden_ssn.csx

int day = int.Parse(ssn.Substring(0, 2));
int month = int.Parse(ssn.Substring(2, 2));
int year = int.Parse(ssn.Substring(4, 2));
int ruleNumber = int.Parse(ssn.Substring(6, 3));

int birthYear = 0;
// Rule 1. 500–749: 1854–1899
if (year >= 54 && ruleNumber >= 500 && ruleNumber <= 749)
{
  birthYear = 1800 + year;
}
// Rule 2. 000–499: 1900–1999
else if (ruleNumber < 500)
{
   birthYear = 1900 + year;
}
// Rule 3. 900–999: 1940–1999
else if (ruleNumber >= 900 && year >= 40)
{
   birthYear = 1900 + year;
}
// Rule 4. 500–999: 2000–2039
else if (year <= 39 && ruleNumber >= 500)
{
   birthYear = 2000 + year;
}

var birthDate = new DateTime(birthYear, month, day);