Maybe if a thread hasn't been posted in for 5 years, or if it was an ask thread for a member who has since been banned. Those are the only cases in which I think this might be acceptable.
Imo a year or even two is too soon to lock a thread. It prevents a newer member from being able to weigh in on a topic, which is just unfair to them. And a new post in an old thread can start discussion back up, which isn't a bad thing. Plus, a new member may have the same or a similar question posed by a thread, but since that one's been locked, they would have to make a new thread, which clutters the forum worse than if the topic had been left open.