As a practical matter, this cannot be done. CGI/Perl programs are not "running" in the background... constantly checking the age of each open ticket or doing anything else. They only execute when users and operators click on something specific.
"Cron" can be used depending only if the hosting account allows such things. Only then can a program be executed on a recurring schedule. But you'd have to write a custom TTX module that searches all open tickets, determines their age since update, and send email as required.
I don't think there's been any demand for such a feature because you can easily look at your ticket browser and see the age of all open tickets in front of you.
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