Calculating age in a form

I entered the following expression into the control source of an unbound text
box on a form. The form was created from a personal info table which DOB came
from, but Date of X came from a different table. When I try to enter this it,
all I get in the text box is #NAME?

=DateDiff("yyyy", [DOB], Surgery![Date of
X])-IIF(Format([DOB],"mmdd")Format([X!Date of X],"mmdd"),1,0)

My first attempt at this involved a query with the same expression, and it
returned correct values. However, when I made the field from the query the
control source of the text box in the form, it also returned #NAME?

I'm guessing there must be something wrong in the way I am going about this.
Any help would be much appreciated.