It depends on the situation. If you have a safe yard and will be there to make sure they don't get stung by bees or other unforeseen accidents, then I'd say "yes". There is nothing sadder than an "indoor-only" cat that accidentally gets outside because they often turn into pure panic
Just use commonsense! My vets are advocates of indoor/outdoor but we are in a rural area where large, fenced yards with guard dogs are common.
It is NOT okay. Your kitten has no protection at this point, and there are countless viruses that can and will take him down fast.
Mega nubers of random viruses are everywhere, especially in the grass.
Then there are the biggies, extremely common and highly contagious.
Then there is panleukopenia. Kills kittens 100%. It is the feline version of parvo in puppies, and who hasn't heard of that and how common it is and hard to kill virus.
Gets on the grass, into the seed, comes back up in full swing in the spring.
Never ever let a young not fully vaccinated kitten outdoors.
I agree with @catwoman707. They won't even keep kittens at the shelter I work at because of how easily it is for them to develop diseases. The majority of them are left in foster care. Also, I'm not sure what kind of natural predators live in your area but I know we get a lot of hawks and coyotes in our backyard that could easily take a grown cat away let alone a kitten!
You have advice, mollomint, from several posters here.
Do not allow 11-week old unvaccinated kittens outside. They can come in contact with other cats who can spread any disease. Other cats may try to hurt them or kill them. There are all kinds of dangers in the outdoors and 11-week old kittens are pretty much defenseless at this point. Keep the kittens inside at least until they're older, can fend better for themselves, and have their shots against diseases.
Eleven weeks is way too young. Along side diseases mentioned above, which can be life threatening and costly on the wallet in the long run, cats need a maturity and months spent in windows to be able to get a visual map in their head of "home" and the area of "home." Young kittens do not have this maturity. They also place adventure over safety, can get lost easily (and most times forever).