DateDiff Error

I have two identical forms, each with a DOB field and an Age field. I'm
using the following expression to display the age in each form:

=DateDiff("yyyy",[DOB],Now())+Int(Format(Now(),"mmdd")<Format([DOB],"mmdd"))

On one form, it works perfectly. In the other form, I get a #Name? error.
Why would it work in one form, but not the other? I was wondering if it was
a formatting difference for the DOB field, but that isn't it. Thanks for
thelp.