If you are using the AGE function I guess you have Version 8.
A calculated field with something like this formula should do it for you:
would give you the number of DAYS for the difference, assuming that your calculated field was set up to be a numeric field.
DATE() will give the current date.
the final parameter (4 in the example) establishes the unit of measure required. Days in this case.
There is more information in the HELP section for the AGE function.