By the sounds of your question, you are tying to implement a validator, if the user hasn't changed the value of dateOfBirthPicker, then highlight it in red?
Instead of setting:
this.dateOfBirthPicker.Value = DateTime.Now;
When you load the form set a global variable to DateTime.Now, set your datePicker value to this variable and use it to compare in your function above:
_dateNow = DateTime.Now;
this.dateOfBirthPicker.Value = _dateNow;
Then do the comparison like this:
if(dateOfBirthPicker.Value == _dateNow)
{
dateOfBirthTBL.Text = "Date of Birth: *";
dateOfBirthTBL.Foreground = new SolidColorBrush(Colors.Red);
dateOfBirthTBL.FontWeight = FontWeights.Bold;
}