Hello, Paul!
I have attached a simple product that should do what you want. Import the product, and then visit the product settings (MarkFL: Auto-Close Old Threads v1.0), and enable the product, set the "Threshold In Days" (the number of days that must elapse since the last reply to a thread before it is considered old, and auto-closed), and select the forums in which the product will be active (child forums are included for ease of selection).
A daily cron job is run to auto-close all threads meeting the selection criteria.
If there are any issues (I tested it), or if you want added features, please let me know. :up: