Forum signature links may do some helps for SEO. But some forum members may stop posting threads or posts for a long time. My question is: Since no more new posts for those members. Do those old signatures still good for SEO? Thanks.
As long as the thread remains indexed, you should get credit for the links.. but as the thread becomes old it usually falls back into the archives. For example, the Digital Point Forums -> Marketing -> Search Engine Optimization forum page has links to pages 1, 2, 3, 11, 51, 101, 501, 1001, etc across the top... While your thread/post is linked to from page 1, 2, or 3 of the Search Engine Optimization main forum page, the engines can easily find your posts. It's within 1-2 clicks (links) away from page 1 of that forum topic. But if you are at page 123 after some period of time and the thread containing your sig link did not obtain backlinks from other sites while it was on pages 1, 2, or 3 then in order to find that thread the engines likely have to follow the link to page 101 from the main Search Engine Optimization forum page... then from page 101 follow the page 111 link... then from page 111 follow the page 121 link... then from there follow the page 123 link to get to the page where the thread containing your post is listed. As you can see, the link to the thread listed on page 123 is now MANY hops away from the main Search Engine Optimization main forum page. The likelihood of that page remaining indexed is slim to none unless, as I mentioned before, it obtained links from other sites at some point (typically while it was a hot topic on page 1, 2, 3). So similar to blog post when they drop off the home page into the archives... and similar to article submissions when they fall off the "new articles" page into the archives... when forum posts fall off of the main forum pages into the archives, chances are good the page will eventually be deindexed. If the page where the sig link is located is no longer indexed, then the sig link is not going to be counted as a link back to your site. So to answer your question... yes, they typically degrade (i.e. go from being counted to NOT being counted) over time.
They most definitely degrade and quite quickly too. If you look at the backlinks Google shows in Google Webmasters, you'll see swathes of DP backlinks disappearing every few months as the pages on which your links are placed get buried deeper and deeper away from the DP home page.