Any responsible & reputable breeder will  insist that your Pet(s) are neutered ideally between 5 and 6 months of age

Some breeders actually neuter kittens before they leave, personally I believe you should wait until they are at least 5 months old.

before the hormones kick in and prior to adolescence

 it is better and kinder to do this if you want a healthy happy pet. 

Please understand that you are not taking anything away,

you are giving them an easier, happier life without the stress of wanting a mate.

IMPORTANT REASONS FOR NEUTERING 

PET KITTENS OR CATS

FEMALES

Cats only Ovulate by stimulation (e.g. mating) and if left to
come into "call" (season)
and not mated, the eggs remain in the ovaries.
 With each call the unfertilised eggs build up until they are like a bunch of grapes in the ovaries,
with the risk of cystic ovaries and subsequently pyometra which if not treated or discovered in time will KILL.

I once had a female siamese who did not come into "call" however over around 6 month period
 she started getting fatter and fatter, and several visits to the vet who could find nothing wrong with her, subsequently one morning I found her dead on the floor surrounded with greenish coloured pus - the infection that no one knew about was completely internal and she showed no symptoms nor signs of infection and was healthy and happy in herself - right up to the day the poor soul died.
A Warning to anyone considering leaving a pet female un-neutered, 

if you are short of money go to the Blue Cross!

MALES

Different breeds including mixed breed or  a "moggy" mature at different rates, 
as a rule of thumb, 6 months is the latest & best time to neuter him,
stopping the hormones developing.
If you have an indoor cat, do yourself and the kitten a favour and neuter by 6 months of age.
This will in the majority of cases prevent "spraying" (marking  his territory)
Cats are the most territorial animal on the planet and eventually he will mark your home,
walls, furniture, just about everything - caused often by strangers coming to the house,
stressful situations such as annual holidays etc.,
workmen; e.g. plumbers, meter readers, decorators etc. etc.

Also of course if you allow your cat outside and he is "entire" he will travel miles to find a female, usually ending up dead in the road having been hit by a car.
Also of course, badly injured, caused by fighting with other males -
territory is of the utmost importance to a cat, and they will fight until one finally gives in
 and tries, injuries permitting, to run away.
Vet bills can run into thousands of your precious pounds.
Also if you love your cat, and he is neutered he will only be allowed outside under supervision
and not left to roam.

To re-iterate my comment above, if you are short of money, 

the Blue Cross will normally help you,
especially neutering male cats.