Assuming that your volhistory is not purged periodically a record of every time a tape is added to or removed from an stgpool will be recorded. A query like this will give you the volume list in order of date used. These would be amongst your oldest used tapes.
select volume_name as "Volume ",date_time as "Date/Time " from volhistory where type='STGNEW' order by date_time
Volume Date/Time
------------------ -------------------------------
T01042 2006-02-28 13:01:03.000000
T01043 2006-02-28 13:01:07.000000
T01044 2006-02-28 16:00:56.000000
...
Similarly, you can query to determine the number of times a tape was added to a stgpool, and that could very loosely correlate to an approximation of how often it was used. This is not the same as the number of times it was mounted. it also does not include the number of times the tape may have been used for a DB backup, export, DBSnap etc.
select volume_name, count(*) from volhistory where type='STGNEW' group by volume_name order by 2 desc
VOLUME_NAME Unnamed[2]
------------------ -----------
T01050 38
T01801 37
T01088 34
...
Or you can simplify it even more to this to give a raw count of the number of times a tape is recorded in the volhistory table.
select volume_name, count(*) from volhistory group by volume_name order by 2 desc
Of course you would then need to determine which of these tapes are empty so they can be ejected and replaced with tapes using a different volser.