I'm not sure why you have the problem, unless you specified date
incorrectly.

DELETE VOLHISTORY TYPE=DBB TODATE=TODAY-5

deletes all DB backups older than 5 days.  
TSM won't let you delete the most recent DB backup.
And, if you are using DBSNAPSHOT  to create the DB backups, you have to use
TYPE=DBS instead of DBB.

But the command above works fine here.  We are TSM 4.1.3 on AIX 4.3.3.