It would have to be a script that closes the thread three months after its last post, wouldn’t it? My sense, anyway, of the three-month rule is that if a thread hasn’t attracted any contributions for three months, it’s presumed dead. I can conceive of a thread that would garner discussion intermittently for several months, so locking threads by start date would kill a still-viable thread.
Of course, if everyone read the FAQ before posting, this stuff wouldn’t happen, right? 