How do I include formula result in another formula?

How do I include formula result in another formula?

I have a field "Age" in my application that is a formula. It calculates a persons age based on their Date of Birth.

I have created another formula field call "Member Lookup" that produces a string to help me make sure I select the correct member in a drop down list. Right now this formula is:

Surname + ", " + First_Name + " (" + Date_of_Birth + ") " + Sex + ", " + Suburb

It returns somthing like this:

Worth, Nicole (27-May-1991) F, Edinburgh

I would like to include the value calculated in the field "Age" but when I do that, I get the error "The variable Age has not been declared. What do I need to add to my formula to declare this variable?

Thank you,

Dave