For the record, director Tim Miller is 49-years-old, not what I would consider "old," but it's certainly true that ones opportunities to direct movies are limited, and Miller wants to be sure that he uses the opportunities left to him wisely. If Miller were to commit himself to a Terminator trilogy, he'd end up spending the better part of a decade in the world of Terminator and he says there are a lot of stories to tell, so he seemingly doesn't want to get stuck telling just one.