Mentioned the Dazed & Confused article about GIFs a few posts back. Here are photos of the article that can be read on the web--kind of a steampunk concept.
As for "all the really GIF heavy hitters are in their 20s," that probably seems true if you have a gnat's eye view of history. Some of the artists in the GIF Show (2006) may have been that age but few are now.
If we must generalize about age, from interactions on dump.fm I'd say the GIF heavy hitters are in their teens.