@yukkysaito - I think the field you’re referencing is “detection_level” which is meant to indicate whether this object has been “detected” or “tracked.” From the notes in http://docs.ros.org/api/derived_object_msgs/html/msg/ObjectWithCovariance.html, here are the definitions:
A Detected object is one which has been seen in at least one scan/frame of a sensor. A Tracked object is one which has been correlated over multiple scans/frames of a sensor. An object which is detected can only be assumed to have valid pose and shape properties. An object which is tracked should also be assumed to have valid twist and accel properties. The validity of the individual components of each object property are defined by the property's covariance matrix.
classification_age indicates the number of “scans” or “detections” made of the object where the classification type is the same. When a sensor classifies an object, it usually tells you how many “scans” of that object have been sent since the object was classified as that type. This helps determine the certainty of the classification.