When does it develop?
Imitation develops immediately from birth on (for instance they protrude their tongue when you protrude yours) and becomes more and more complex with age (for example, at 9 months they press the same button to obtain the same sound, or they join their hands to clap like the other does).
The answer depends on which form of imitation you have in mind.
A newborn is able to imitate facial gestures and sound, at 2 months hand gestures, at 3 months, trajectory towards a part of the body and after 6 months simple actions (when grasping is mastered).
After 6 months, s/he will imitate for instance ‘grasp a spoon’. But is s/he able to turn the spoon inside a bowl? This action requests to enchain two simple actions: first to take the spoon, second to turn the spoon inside the bowl, the whole being possible if the infant coordinate both hands, one maintaining the bowl and the other turning. This will be possible after 14 months.
Enchaining many actions to perform a novel goal after observing without doing will take place much later, at 3 years.
What are the implications of this skill for a person's development?
Imitative behaviour allows people learn the typical use of certain objects, acquire new skills and obtain the necessary basis for the development of communication.
The importance of imitation in development is crucial. Indeed, imitation has been shown to contribute develop many functions among which self-recognition and body part recognition, communication without words, joint attention, empathy, self-other relationship, TOM, and learning via observation.
What difficulties do people with autism and intellectual disability have in developing this skill?
Many people with Autism and intellectual disability do not develop imitative behaviours at the evolutionary moment that it would be necessary. As a consequence, they have difficulties in other areas that are also basic for their development such as play skills, peer interaction or joint attention.