8 weeks is my cut off point too. I did tame down a pair who came to me age 10 weeks but it took over 4 months.
They were/are timid adults at the end of it but luckily they were strikingly beautiful long hair tabbies and were eventually adopted by a lovely lady but she had to put a fair amount of work into them as well.
She is the only one they will come near and she's had them for 3 years.
I know my views are unpopular with some but I believe if a stables or small holding are prepared to care for and watch over them, if they are over 12 weeks of age when trapped I get them neutered and released back. Wouldn't do it if they don't have anyone to care for them, mind you, but some stables homes are every bit as good if not better than a lot of the poorer indoor homes.
I recall a whole litter (3 gingers, 1 black) that came to me just too late... at least 10 weeks. We tried for a fortnight and got nowhere.
A farmer lady approached me looking for mousers and was prepared to put in the ground work settling them. They went to live with her and are having the time of their lives chasing mice out of the hay barns. They even come running when she calls them for supper.
They won't come closer than 4 feet from her though.
How long would kittens like that stay in a rescue pen?