From the virtual server page, click the Content Management tab.
Click the General sub tab.
Go to the Compression > Precompressed Content section and select from the following options.
Age Checking — Specifies whether to check if the compressed version is older than the non-compressed version.
If selected, then the compressed version, even if it is older than the non-compressed version, will not be checked.
If selected, then a Vary: Accept-encoding header is always inserted when a compressed version of a file is selected.
If not selected, then a Vary: Accept-encoding header is never inserted.