Standard advice is to neuter at 4 months. Any vet saying six months is very out of date. Any vet saying to wait until 17 months should be struck off!
I work in a specialist neuter clinic and we neuter all our kittens for homing and neuter/returns as long as they weigh 1kg - so around 9 weeks old.
Indoor only females MUST be neutered. A female in heat will do anything to get out - including throwing themselves out of windows of flats on a 10th floor or more. We see many injuries and deaths in our clinics because of this very thing.
A female which keeps coming into heat but can't mate becomes psychotic. They end up so mentally disturbed they become very, very aggressive. We have known quite a few that have put their owners in hospital they became so aggressive. Once they were neutered and the hormones had a chance to calm down they became friendly and docile again.
So OP you are very wrong to think that it isn't necessary to neuter an indoor only female. I don't care how many animals you've had in the past, if you didn't know that then yes, you are ill informed.
If money is an issue there are a number of voucher schemes from various charities such as Blue Cross, RSPCA and CP.
And if you are in London you can come to us, Celia Hammonds, for only £25 or, if you are on benefits, for free.