Assuming your first textbox contains the date of birth use:
Protected Sub txtDob_Enter(ByVal sender As Object, ByVal e As EventArgs) Handles txtDob.Enter
if isdate(txtBirthDate.Text)
txtDob.Text = DateDiff(DateInterval.Year, Now, CDate(txtBirthDate.Text)).ToString
End sub
Every time you enter / focus your second textbox, the difference between current time and day of birth will be re-calculated.
This is VB.NET code, but it can easily be translated to C#.