how do calculate age from date of birth

Simplest solution to the problem:
First set the dropdownlist to autopostback and add eventhandler for OnSelectedIndexChanged
In your eventhandler, get the SelectedValue of the dropdownlist and convert it to int(year)
Then do subtract the value from DateTime.UtcNow, and subtract the result.Years to the txtBox.Text property.

Refer this link,
Here they are using 3 Textboxes
http://www.visual-basic-tutorials.com/age-calculator-in-visual-basic.htm[^]

and here by using Date String,
http://stackoverflow.com/questions/16874911/compute-age-from-given-birthdate[^]

You should use a code similar with the next C# code:

int GetAge(DateTime birthDate)
{
  DateTime temp = DateTime.Now.AddTicks(-1 * birthDate.Ticks);
  return temp.Year;
}

calculate using TimeSpan like....

DateTime dob = .....
DateTime Today = DateTime.Now;
TimeSpan ts = Today - dob;
DateTime Age = DateTime.MinValue + ts;

subtraction due to min value

int Years = Age.Year - 1;
int Months = Age.Month - 1;
int Days = Age.Day - 1;