Old discussions become neglected discussions.
Neglected discussions are filled with outdated advice.
If most of your new visitors arrive via the long-tail of search, your community could inadvertently send most of your audience to outdated information.
There is a manual and automatic solution to this.
1) Periodically review and update discussions.
You can’t keep every discussion up to date. But you can go through the top 50 landing pages of your community via Google analytics and check the information/solutions remain accurate. Better yet, ask for volunteers who might like to help you go through the top 500.
2) Let members highlight outdated information.
StackExchange have an option for members to improve answers from old discussions. Other communities allow members to flag outdated information. A simpler manual solution is to ask members to periodically flag outdated information and enable a simple option alongside or within discussion posts to highlight bad material (pop-up warning boxes on discussions more than 1 year old also work).
3) Add useful contributions to a database.
Document and add information to a central database of information. Instead of relying on keeping old discussions up to date, simply tag and properly categorize new information and try to link discussions to that section instead. Over time search should naturally push people to this information.
4) Remove, review, and update old discussions.
Automatically remove old discussions or set up a system to review discussions after several months. Periodically flag discussions from a year ago for a further review. Decide to remove redundant information, update information, or revamp everything.
Done right you continually keep discussions fresh and content relevant to all members. You can give most members the answer before they even ask the question. That’s pretty powerful.