I was adopted by two siblings when they were 18 months old. I didn't pay for them - they were just needing to be rehomed.
I would be slightly suspicious if someone wanted to sell older kittens or cats.
Frankie (13½ years old) is still with me but his sister died at the beginning of November. They, at best, tolerated each other but it was often all out war. Princess Merlin was a stroppy bully.
I am not against anybody buying pedigree cats or dogs but if you just want a moggie a rescue centre is usually a good place to go. Our local Cats Protection branch is excellent.
Neutering - there is no hard and fast rule, it depends on the vet who will be doing the operation. But the girls can get pregnant at a very young age.