Я нашел решение, и оно работает для меня.
public static DateTime GetDateOfBirth(int year, int month, int day)
{
var today = DateTime.Today;
int currentYear = today.Year;
int currentDay = today.Day;
int currentMonth = today.Month;
if (day >= currentDay)
{
currentMonth--;
currentDay += DateTime.DaysInMonth(currentYear, currentMonth);
}
if (month >= currentMonth)
{
currentMonth += 12;
currentYear--;
}
return new DateTime(currentYear - year, currentMonth - month, currentDay - day);
}