A cat's entire capacity for socialisation depends upon the quality of the attachment it has formed with its mother, and on the degree of socialisation of the mother herself.
A cat develops much faster than a dog: where a dog takes about six months to become autonomous, the key period for a kitten to grow up into a well-balanced cat lies in the sixth week of life.
Of course, all is not lost, and it is still possible to enhance socialisation at the usual age of adoption: i.e. around two months. But it needs to be borne in mind that cats are not always necessarily social creatures and that certain cats which have gone back to their wild state are capable of spending their whole life without any social interaction at all outside of mating seasons.
In the life of a cat, the first period of attachment is primordial and this capacity for attachment, which is a juvenile characteristic, needs to be preserved. If this first attachment is of a quality such as to allow your cat to feel confident with all the humans and often the other species of animal (such as dogs) which it may encounter, then it will be relaxed and glad to be with you. Otherwise, it will become familiar with just one or with just a few persons, and hide away whenever a stranger appears; it will always be chary of the unknown, although your presence can reassure it. This is the distinction between a social and a merely familiar cat.
If such a lack of socialisation is making your cat aggressive, do not hesitate to consult your Vet.
It is possible to detect disturbances in the process of detachment by means of the carrying reflex referred to above. One also often finds cats which go on trying to breast-feed, using pieces of cloth or, more commonly, their owner for this. From time to time, the animal forcibly seeks contact, settling on or against its master and taking a finger or some hair in its mouth to suck on for a fairly long time. In itself, this behaviour has nothing necessarily pathological about it, but you should talk to your Vet about it if it is associated with other signs (such as fearfulness, growling or spitting on contact, or on the other hand too quiet a cat...).