Age At Time Of Test Date

I am trying to create a field that will calculate a persons age at the time of a test date. I have the DOB field that gives for example :

3/10/1993

Then I have a field that turns this into the Age in Year and months

14 yrs 0 mos

I have a TEST Date field that tells me when they took the test:

10/1/2001

Now I need a field that will calculate how old they were at the time they took the test. I tried using this formula:

AgeAtTest([DOB], [TestDate])

but then in my field I am getting this : #Name?

It should calculate them to be: 6 yrs old at the time of the test. How can I make this happen?

Thanks!