First of all the title isn't very clear of what you really want to ask.
Second, consider casting your date to a string before putting in the Text property of txtAge.
Third, you should use the DateDiff function to determine the days? months? years? between the two dates.
example:
System.TimeSpan diffResult = DateTime.Now.Subtract( Convert.ToDateTime(txtdate.Text));
txtAge.Text = diffResult.Years.ToString();
Good luck!