1. An activity meter comprising: an activity amount acquisition unit arranged and programmed to acquire an activity amount of a user; an index acquisition unit arranged and programmed to acquire an index relating to the activity amount; and an index output unit arranged and programmed to output the index acquired by the index acquisition unit; wherein the index acquisition unit: includes an activity age acquisition unit arranged and programmed to acquire an activity age represented by a standard age of a person who does a same amount of activity as an activity amount acquired in a unit period, using body information of the user; is arranged and programmed to acquire the index to enable the active age to reach a target activity age, based on the activity amount acquired in the unit period; includes a target activity amount acquisition unit arranged and programmed to acquire a target activity amount representing an aggregate amount of activity done in the unit period by a person of the target activity age; is arranged and programmed to acquire an index, using the acquired target activity amount; compares an activity amount of the user acquired by the activity amount acquisition unit within the unit period with the target activity amount acquired by the target activity amount acquisition unit, and acquires the index based on a comparison result; and computes a difference between an intermediate activity amount representing an activity amount from a start of the unit period acquired by the activity amount acquisition unit to partway from the start of the unit period and the target activity amount acquired by the target activity amount acquisition unit, and acquires the index based on a computed difference; the unit period includes a plurality of time slots; the activity meter further comprises a storage unit arranged to store a representative value for each time slot of the activity amount acquired by the activity amount acquisition unit in association with the time slot; and the index acquisition unit: includes a predicted activity unit arranged and programmed to acquire a predicted activity amount in the unit period, using the representative value stored for each time slot in the storage unit and the intermediate activity amount; and is arranged and programmed to acquire, as the index, a predicted activity age representing an activity age acquired using the acquired predicted activity amount and the body information of the user.