Calculated fields were introduced in Act! with version 16, but they didn't always work as expected with certain databases. The cause for this is based on Act!'s unique database design that employs spillover tables to manage constantly expanding database schemas.
Shortly into the release cycle of Act v16 it was discovered that if a database had too many spillover tables (typically ten spillover tablles or approximately 300 custom fields), date based caclulated fields did not work properly. That is to say if you had a field that caclulated the number of days or years from-or-to a particlular date field, the field calculting the number of days or years from that date would never update. An Age Field, for instance would not update the contacts age even when a birthday passed.
What we discovered is that this behaviour would only present itself if there were too many spill over tables in the database, AND the two fields in question were not in the same spill over table. In other words, if the age field and the date of birth field were in separate tables, they age would not calculate.
Create a new Birthday field and age field so that they end up in the same table. Once this is done the age field updates as expected, and the database information is accurate.