The added or subtracted value results in un-representable Date Time. Parameter name:T
Dim offset = New Date(1, 1, 1)
Dim dateOne = dtpbdate.Value
Dim dateTwo = Date.Now
Dim diff As TimeSpan = dateTwo - dateOne
Dim years = (offset + diff).Year - 1
If years < 18 And years < 50 Then
MsgBox("Age requirement must be 18 - 50 years old!")
Else
txtage.Text = years.ToString
End If