Why no longer than 60 days?
Paging through popular forums can be a big source of site timeouts. For the system to show you topics from 100-150 days ago, it must pull and sort, in order of date, all topics in the last 6 months in that forum. That can take an excessive amount of time if the site is also busy, and during that time, nobody can post/edit/reply at all.
It is possible to search for items older than 60 days. See this section for more information.
Please don't reply to old posts.
Going back and reading them is fine, and enjoyable, but please do not attempt to respond to an old thread. It is much better to start a new one.