What is the Last Updated field and when is it updated?
The last updated field is updated when a work item has changed. This change can be as a result of a direct edit by a user, or it can be as a cascaded operation from something else being updated, such as a child being added to a parent work item. It can also be updated as a result of an automated calculation service such as the portfolio rollup to update the percent complete fields.
You can manually trigger a rollup calculation using the gear menu next to the percentages.