UNIX find: opposite of -newer option exists?

If you only need files that are older than file "foo" and not foo itself, exclude the file by name using negation:

find . ! -newer foo ! -name foo

You can just use negation:

find ... \! -newer <reference>

You might also try the -mtime/-atime/-ctime/-Btime family of options. I don't immediately remember how they work, but they might be useful in this situation.

Beware of deleting files from a find operation, especially one running as root; there are a whole bunch of ways an unprivileged, malicious process on the same system can trick it into deleting things you didn't want deleted. I strongly recommend you read the entire "Deleting Files" section of the GNU find manual.

You can use a ! to negate the -newer operation like this:

find . \! -newer filename

If you want to find files that were last modified more then 7 days ago use:

find . -mtime +7

UPDATE:

To avoid matching on the file you are comparing against use the following:

find . \! -newer filename \! -samefile filename

UPDATE2 (several years later):

The following is more complicated, but does do a strictly older than match. It uses -exec and test -ot to test each file against the comparison file. The second -exec is only executed if the first one (the test) succeeds. Remove the echo to actually remove the files.

find . -type f -exec test '{}' -ot filename \; -a -exec echo rm -f '{}' +