Seems to me that both keeping older projects around and having some indication of recency would have their own use. What I think I would do is mark projects older than 6 months with an 'inactive' label, maybe grey them out and place them at the end of the page if they're not already organized by date. Sent out automated emails to the people posting them a week or two before this happens, when it happens, and maybe one reminder a week or two later, allowing them to reactivate old projects in a click and encouraging them to update anything that's become outdated.