Lacan gives six months as the earliest age at which the experiences he categorizes into the mirror stage can occur. He describes the infant's fascination with her image in some detail, emphasizes the child's tendency to strain forward, as if to get closer to the reflection. This physical straining will become emblematic of the psychological strain of the individual later in life.
Eighteen months serves as the cut-off point for the mirror stage; when children begin to acquire language and form social bonds with others, the mirror stage, as a developmental phase, comes to an end, but the dynamic relationship between the subject and his image remains a perpetual force in subject's psychic life. It is a libidinal relationship--the psyche invests libido in the image of itself, along the lines suggested by Freud in On Narcissism: An Introduction. Having an image of oneself is part of human reality--what Lacan calls an "ontological structure of the human world." We are always, in a sense, "someone else" insofar as the image is perceived as separate from us.
By introducing the idea of identification,, Lacan develops his mirror stage model as a process in which the infant does not simply "match up" what he sees in the mirror with his experience of his own existence, but is rather transformed by the vision of his image, which he "takes on," both as a representation of himself and as a goal to which he strives. Lacan makes an allusion to Judeo-Christian theology here: Genesis describes humans as made in the image of God--in the imago Dei--and this idea has informed both Jewish and Christian philosophy. Augustine, for example, suggests that humans must strive to restore the divine image in themselves. Lacan's mirror stage can be understood as an ironic version of this theology: to be human is to be the creation of one's own image.
It is important to note that during the year or so of the developmental mirror stage the child is physically helpless, unable to move about easily on his own, unable to feed or to care for himself. To some extent, the child experiences his body as incomplete, even fragmented. The image in the mirror, however, is the image of a whole body, a singular, complete entity. The child's encounter with an image that does not in fact correspond to his experience lays the foundation for what Lacan calls the "exemplary function" of the mirror stage. Throughout life, the image of ourselves that we hold in our minds fails to correspond completely with our actual physical, historical, emotional being. The child will go on to elaborate a complex "self-image" based on his relations with other people as well as on the conventions of language, which will give him the ability to name himself and to refer to himself with the pronoun "I". The difference between the projected image of himself and his actual self, however, will continue disrupt his experience of his own existence in various ways.