Since there is no way for the plugin to guess which files you want monitored, you're going to have to set each file up separately. Put the following in a file in your plugin-conf.d directory.
[file_age]
user root # May not be necessary, depending on which files you want monitored.
env.file1_path /var/log/syslog # Mandatory, complete path to file.
env.file1_label System syslog # Optional label if you don't want the file name to be displayed.
env.file1_warning 86400 # Optional warning level. Measured in seconds. 86400 is one day of seconds.
env.file1_critical 864000 # Optional critical level. Measured in seconds.
Continue with file2, file3, etc...
Here, have some seconds:
3600 One hour
7300 Two hours
10800 Three hours
21600 Six hours
43200 Twelve hours
86400 One day
172800 Two days
259200 Three days
604800 One week
=head1 AUTHOR
Edward Plainview <it@sverigedemokraterna.se>
=head1 DONATIONS
If you wish to donate money for this plugin, please read https://it.sverigedemokraterna.se/donera/