Modules & VBA :: Negative Numbers In Calculations

Ok i know negative numbers can be a bugger in calculations:

Code:
Function FEV1pred(height As Double, ClientID As Double) As Double

Dim a As Double
Dim Age As Integer
Dim DOB As Date
Dim TestDate As Date
Age = DateDiff("yyyy", DOB, TestDate) + CInt(Format(DOB, "mmdd") > Format(TestDate, "mmdd"))
'In this case the clients Age is 39
a = -0.0244

'Now for the fun stuff...

FEV1pred = a
'returns -0.0244 so i know the dim a holds the neg number
FEV1pred = (a * Age)
'returns 1.708 (which is incorrect, how this calculation is reached.)
FEV1pred = (a * 39)
'returns -0.9516 (which is correct)
End Function

So something about the dimming of Age seems to be doing something to the equation but i can't work out what?