HI Tristan,
In order to achieve this you would need to build a calculated field that captures a timestamp when the status of a project is set to current.
In the below example I have created a calculated field on a project custom form and called the field 'Current Timestamp'. Using the following code, Workfront will watch to see if the project status is set to current. If it is, it will check whether or not the timestamp field is empty and if not, it will return a timestamp field of now.
IF(Status='CUR',IF(ISBLANK(Current Timestamp),$$NOW,Current Timestamp),Current Timestamp)
Once you have captured a timestamp of when a project was set to current, you could then calculate the project age by carrying out a datediff calculation between the 'Current TImestamp' field and now.
One thing to point out is that this will only work for new projects going forwards, you can't retrospectively calculate when a project was set to current.
Hope this helps!
Best Regards,
Rich.