While you can let them go by 8 weeks, its much better to keep them till around 12 weeks (my rex kittens never left my house till 16 weeks old). At 8 weeks they are barely weaned and litter trained.
Mentally and socially they need time to interact with siblings, mom, and other adult cats. If taken too soon, they sometimes have social problems. And at 12-16 weeks old they are still very much kittens.
I kept my till 4 months old to be sure they all had their shots, were very well house trained (no scratching furniture, climbing curtains), and ready socially with other people and cats.
Our barn kittens (last batch) went to new homes around 12 weeks old.