This is an excellent idea. You're right, there are too many "Similar topics". Is it possible though in addition to the time limitation, to do a coding so that those threads of a longer time ago that are longer than two or three pages are included, as there are some really popular threads of a long time ago, that can be identified with multiple pages. I'm for example thinking about the competition of the Philosopher Princess at the end of 2006, that resulted in one of probably the best threads of the Philosophy and Religion forum.
In many cases we don't lock an old topic because the discussion can still be relevant, but in some cases we do when it is clear it will never be relevant again due to, simply, time.
It's better to have few guys reply to an old topic than having all of them locked after a certain period, because it's (still) quite difficult to code a system which will recognise a good relevant discussion from not a good one, I think, and the number of pages has nothing to do with that.
Well not displaying them isn't good either, since if there's an old topic and it holds good discussion potential, then it would be a good idea to 'bump' it to the front page.
I would think that in the coding forum in particular, it might be helpful to have links to very old, but related topics, since those related topics might be about the very same problem.