As @schoenitz points out, it is already possible to sort by oldest date updated via the Identify filters.
It is also possible to do this while Exploring observations by manually adding
&order_by=updated_at&order=asc to the URL.
Since as @cmcheatle points out the default sort order is not likely to be changed for the foreseeable future, I’m closing this topic to redirect discussion back to the previously referenced feature request, so we don’t fork the discussion and make it harder to follow.
Questions of how to define the “age” and taxonomic rank to which to confine any automatic IDs would best be addressed there.