I've been on the net for a few decades and active in a few hundred forums. In my experience it is rare (and very cumbersome) to have rules that lock old threads (aka necromancy blocks). It is also not wise, e.g. suppose someone discovers a safety issue with some cells someone raved about long ago. Then there is no way to update the old thread with warnings (and google page rank may elevate the old thread much higher than any new threads in search results).
So my recommendation is to not have such limits. Or if one insists, then limit the blocksto newbies or somesuch.Their detriments arefar worse than theiradvantages.
Not too infrequently I update my old posts with new info (even some from over 10 years ago on forums that have managed to survive that long).
Generally, locality of information is something one should strive for since it greatly aids efficient comprehension (not to mention many folks are too lazy to follow links - which I know well since I link heavily - so much so that some folks complained that they got lost following my links, and even somehow managed to get stuck inloops!).