As long as you have verified that the value entered is a valid date and is less than or equal to the current date then convert that value to a date and calculate the difference in years between that date and now. Then you have to subtract one if the birthday falls later than the current date.
Dim birth As Date = CDate(MaskedTextBox1.Text)
Dim age As Integer = DateDiff(DateInterval.Year, birth, Now)
If birth.AddYears(age) > Now Then
age -= 1