How useful is it to study Old LSATS (ex Preptests 7-18)?

Yes, you can still use that book and get high value out of it. The thing about logic is that the basic principles haven't changed in eons, so the ideas they use to make up the questions are the same. There are shifts in what the test makers emphasize over time, but not enough so that taking those old exams would be useless. Plus, compared to the Logic Games you see today, the older games are harder, so you would probably find it useful to face those tougher games as you prep for the upcoming test.