A good age for slaughter?

This is a bit of a broad question, but at what age(ish) do you harvest animals? How old do you let them get before you send them out for slaughter?
Rabbits? Ducks? Pigs? Chickens? Cows? Goats?

I appreciate the info!

Also, this one is probably more for Paul, but how does Sepp Holzer harvest his cows and pigs when they're living in big family groups mostly in the wild? Does he cull the young ones so there aren't too many bulls? I know he keeps them in paddocks, but I remember reading that his paddocks are pretty huge. Does he familiarize the animals with his presence? Any insight would be great!

this year we raised a handful of freedom rangers (meat birds better suited for higher altitude) and we harvested them at 12 weeks.
next year we will harvest the cockerels at 10 weeks, and the hens at 11 or 12 weeks. we will be doing this so the roo's dont get to big (we had a 6.8 lb finished chicken this year!) and it saves a bit on feed.

i believe the cornish X rock crosses are harvested at 8-9 weeks.

more info on freedom rangers: http://www.freedomrangerhatchery.com/learnmore

sorry i am not more help on the others animals, im an interested to hear responses though

We usually start eating chicken around the time the roosters begin to crow, and we eat the mean ones first, to reduce fighting. Generally I process one chicken a week until I run out of eating chickens. I don't tend to freeze them. They just get a little tougher and more flavorful as time goes by, but I usually cook them in a slow cooker anyway so it's not an issue.

We've only done one goat, and it was slaughtered way earlier than intended due to an accidental death - so I can't answer on that one...
For rabbits, ducks, and chickens, we tend to let them reach what appears to be their full grown size (or as close as it appears they'll get, since it varies with the rabbits) and then we slaughter. We haven't done this as a hard and fast rule, since we are just 2 people slaughtering, and sometimes it's hard to kill too many bunnies or any animal at once, but as the guy at the animal auction says, "They don't eat any feed in the freezer." So that seems to always be a good general rule.

Cows/bulls 18-24 months is optimal.
Sheep depends on the size of the breed. My breed (Black Welsh Mountain) is a bit too small for 6 months. I'm thinking of cull ram lambs at a year. Ewes don't get culled till the stop producing lambs or till there are too many.

I do slow Cornish X and I did some this weekend (@ 12 weeks) that were 4 lbs (perfect for me).