Why are you using obsolete VB6 data types?
If you use the DateTime class that has been in VB.NET since 2002 it's pretty easy:
Dim ageYears As Integer = 21
Dim ageMonths As Integer = 3
Dim ageDays As Integer = 22
Dim dob As DateTime = DateTime.Now.AddYears(-ageYears).AddMonths(-ageMonths).AddDays(-ageDays)
Console.WriteLine(dob)