LaunchAgent that runs periodically to clean out files from specified directory that are older than 7 days. Replace PATH_TO_CLEAR with the directory you want files deleted from. Change LOG_PATH and ERROR_LOG_PATH to appropriate files for logging. Remove if undesired.

local.cleardir.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple/DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
    <dict>
        <key>Label</key>
        <string>local.cleardir</string>
        <key>RunAtLoad</key>
        <true/>
        <key>EnableGlobbing</key>
        <true/>
        <key>ProgramArguments</key>
        <array>
            <string>/usr/bin/find</string>
            <string>PATH_TO_CLEAR</string>
            <string>-mindepth</string>
            <string>1</string>
            <string>-mtime</string>
            <string>+7</string>
            <string>-exec</string>
            <string>rm</string>
            <string>-rf</string>
            <string>{}</string>
            <string>\;</string>
        </array>
        <key>StartCalendarInterval</key>
        <array>
            <dict>
                <key>Minute</key>
                <integer>15</integer>
            </dict>
        </array>
        <key>Debug</key>
        <false/>
        <key>AbandonProcessGroup</key>
        <true/>
        <key>StandardOutPath</key>
        <string>LOG_PATH</string>
        <key>StandardErrorPath</key>
        <string>ERROR_LOG_PATH</string>
    </dict>
</plist>