find /opt/app/logs -mtime +XX -exec rm {} \;
For example, to delete files older than 90 days:
find /opt/app/logs -mtime +90 -exec rm {} \;
If the same directory has multiple type of files, and want to delete ONLY those with .log extension:
find /opt/app/logs -name '*.log' -mtime +90 -exec rm {} \;
If you want to see the files to be deleted, run the same command removing "-exec rm {} \;"