You're probably getting the Operand Types error because the field you're using as a date item in the Age function is defined by Monarch as a character field. You can use the CtoD (Character to Date) function to convert the field so that the Age function will calculate properly.
Also, note that you'll need to use the interval value of 4 in the Age function to calculate the number of days when determining the aging.
Finally, your "0 to 30" field will have a formula similar to:
[font="courier"]if((Age(CtoD(MyDate,"y/m/d"),Today(),4)>=0) .And. (Age(CtoD(MyDate,"y/m/d"),Today(),4)<=30),InvoiceAmount,0)[/font][/quote]Other aging fields would be defined by just revising the lower and upper limits.
Does this help you progress?