The amazing @sunnydeveloper & I led SocialCoding4Good at Benetech.org from 2011-14 to do just that, though not just retirees :) Many tech nonprofits need devs yet lack resources to manage individual task contributions.
Space has grown *a lot" since then, lots of collective learning + many new vertical & regional organizers. Not all #opensource but lots of real social impact. Ovio & DataKind are a couple of favorites: https://ovio.org/ www.datakind.org/
@scottjenson @zceline @sunnydeveloper I remember there was a whole open source design group out there (Jan C. Borchardt, is the name that comes to mind, but I may have butchered it). I remember Sandstorm always needed design help but nobody was ever offering (right now we aren't in a position to use that help, mind you, we're a bit stuck financially).
A lot of great projects have neat functionality but are written by people with no hope at making a good interface.