Just save this script and attach it to a smart group via the Info panel. Opening the smart group in its own window or selecting it in split/3-pane/column views will execute the script.
tell application "DEVONthink Pro" to set label of children of theRecord to 1
This script moves all children not modified within the last week to the trash. Attach it also via the Info panel.
tell application "DEVONthink Pro"
set theRecords to children of theRecord
repeat with theRecord in theRecords
set theDate to modification date of theRecord
set theInterval to theDate - (current date)
if theInterval < -(3600 * 24 * 7) then move record theRecord to trash group of database of theRecord