How i convert dataetime to int and string to Int

The code has serious issues it wont even compile.You are totally confused with the conversions. I would suggest understand the basics of assignment operations too.
 
you are trying to store a string data into an integer variable 'age' at the following location which is wrong you never store a string into an int.

//wrong code
age =Convert.ToInt32(ddlyear.SelectedItem.Text).ToString(); //compiler error
//Correct Code
age = Convert.ToInt32(ddlyear.SelectedItem.Text); // is enough

//this code also throws compiler error, 
result =Convert.ToDateTime(datenow).ToString() - Convert.ToInt32(age).ToString();

 
Try this

DateTime now = DateTime.Today;
int age = now.Year - bday.Year;
if (bday > now.AddYears(-age)) age--;

calculate-the-difference-between-two-dates-and-get-the-value-in-years[^]

Your code is rather weird.
You seem to have little understanding of the C# type system[^].
 
You seem to try to calculate a start date/time by subtract some "age" from "now".
What is "age"? seconds, minutes, hours, days, weeks, monts, years, centuries, mayan-cycles ...?
 
Anyways: when calculating with times, you need to understand the difference between
- a point in time (like "now", "yesterday", "last year", etc.)
- and a duration (like "1 hour", "20 years", etc.).
 
Durations can be added/subtracted from each other, resulting in a new duration.
Adding/subtracting two points in dime result in a durations.
Adding/subtracting a duration to/from a point in time gives a new point in time.
 
DateTime denotes a point in time.
TimeSpan denotes a duration.
 
In your code, I assume "age" to be a duration.
1) set the point in time variable "now" to DateTime.Now
2) parse the "duration" input text into a TimeSpan (you need to decide on the right unit)
3) subtract the "duration" from "now"
4) convert the result to the desired point in time format (e.g. year only) and assign that to your text output variable
 
Cheers
Andi
 
PS: Strongly recommend to use TryParse(...) instead of any Convert.XXX(...) or Parse(...) methods. Reason: user input is potentially broken.