Any age works. If they are small they make good fryers.what's generally the age/size one processes a heritage breed? i may have a few at some point, but they're all still fairly young yet... but love the idea of a processing get-together, and perhaps learning on not-your-own bird!
Most people wait until at least 16 weeks. On the Heritage Large fowl of the Basque thread, someone said that after 24 or so weeks they do not grow enough to warrant the extra time and feed.
From 16 to 20 weeks in general but Dorkings and Langshans can go longer and still be good. The older they are the more you need to Brine them and moisture/slow cook them.
I am sure one of us could handle the killing part.