There are really two questions here:

 1. How to convert the string input into a `DateTime` object
 2. How to calculate age once you have your data in the correct format.

I'll let you follow other's instructions for how use `TryParseExtract` which is definitely the correct way to go here. 

When determining someone's age from their DOB, try using this:



    Public Function GetCurrentAge(ByVal dob As Date) As Integer
        Dim age As Integer
        age = Today.Year - dob.Year
        If (dob > Today.AddYears(-age)) Then age -= 1
        Return age
    End Function

It is the vb version of the top answers on Jeff Atwood's very popular question [How do I calculate someone's age](https://stackoverflow.com/q/9/1366033)

I wrote a blogpost about [calculating age from dob](http://codingeverything.blogspot.com/2013/05/calculating-age-from-dob.html) as well.

I have 2 comboboxes and 2 textboxes. My first combobox contains months in this format january, february, etc, and the other combobox contains numbers from 1 to 31. My first textbox is `txtyear`. Once the user input birth year to `txtyear` a variable `BOD` will be equals to this. 

    Dim BOD as string
    BOD = cbomonth.text + "-" + cboday.text + "-" + txtyear.text

The purpose of my last textbox is to handle the age of the user that will be computed when the cursor lost focus on `txtyear`.

Can anyone help how to compute the age.