Work Item Age is the elapsed time between the moment a work item was started and the current moment. Thus, it is only applicable to work items that have not yet been completed.
Work Item Age is also know as a leading indicator to better inspect progress towards the Sprint goal. This usually occurs during the Daily Scrum.
An important role of this metric is therefore to identify PBIs (Product Backlog Items) that are progressing with difficulty and need attention. Other ways to identify bottlenecks include regularly inspecting the Scrum Board. Additionally, the Cumulative Flow Diagram also provides insight into which PBIs need attention.