Convert your date to a DateTime (or use the DateTimePicker and get a DateTime directly - it's a better solution)
You can then just check it:
string date = "2/2/2013";
DateTime dt = DateTime.Parse(date);
if (DateTime.Now >= dt.AddYears(18))
{
}
if (new DateTime(2011, 12, 31) < dt && dt < new DateTime(2014, 1, 1))
{
}