A position statement from the Feline Veterinary Medical Association has explained why some feral kittens respond to socialization better than others.

Strong cat-human bonds are usually forged when kittens are socialized before a certain age. For companion kittens — that is, kittens already used to some human contact — that’s before nine weeks of age. If these kittens don’t have positive interactions with people before then, they may be afraid of humans for the rest of their lives.

When it comes to feral kittens that aren’t used to any human contact, attempted socialization is not recommended after four months of age because of the likely emotional distress. Instead, the association recommends using a trap-neuter-release process, where kittens are captured, neutered and returned outdoors.