Age Norms

Age norms are used to represent typical performance or some aspect of development for children within a particular age group. Used as an indication of the average age at which certain behaviors are expected to occur, they provide a metric against which same-aged peers can be compared. Alternatively, they provide guidelines to determine where along a developmental continuum an individual's skill or behavior may fall. Depending on the measure of interest, these norms may be expressed in various ways.

The use of age norms assumes homogeneity of a group with respect to particular skills or behaviors. Because these can be expected to be normally distributed within a population, age norms can be computed on the basis of the average performance of the individuals within that population. For example, a vocabulary of 50 words is considered to be the norm for typically developing children between the ages of 12 and 18 months. Children whose vocabulary size falls within, above, or below this range may therefore be considered typical, precocious, or delayed, respectively. Age norms exist as well for certain physiological measures (e.g., the pitch of the voice) as well as developmental milestones (e.g., crawling or walking).

Age norms are also employed in characterizing the acquisition or emergence of certain skills. These norms assume an ordering of developmental stages and are often used to characterize motor functions, aspects of speech and language acquisition, social behaviors, and so forth. Often, the emergence of behaviors is considered to be predicated on the acquisition of prerequisite skills, thus implying a fixed and orderly developmental sequence. This pattern would be typical of sensorimotor phenomena such as locomotion and manual dexterity. For example, the ability to stabilize the trunk using large muscle groups typically occurs at a certain age and precedes the development of movements necessary for more precise distal movements. By extension, failure to develop earlier skills would predict the delay, impairment, or absence of later-emerging skills.

Other behaviors may also appear along a developmental continuum. For example, starting as early as 1 year of age, norms exist for the production of classes of speech sounds (e.g., stop consonants vs. fricatives), individual sounds within those classes, the ways those sounds are used in words, syllable structure, and so on. Although motorically complex sounds often appear later than simpler ones, a fixed order does not necessarily apply. Age norms exist as well for the acquisition of grammatical structures and various parts of speech. Failure to acquire speech and language according to these norms is considered grounds for further evaluation or intervention.

Age norms are typically easy to understand, with respect to performance both at a particular age and over time. However, their usefulness is limited to certain types of developmental behaviors or skills. Moreover, although skills or milestones that fall at or within age norms may be considered to be normal or typical, interpretation is more problematic when performance lies outside those norms. As a result, measures are typically standardized so that (a) a child's performance can be characterized with respect to other children of the same age or grade and (b) a comparison of performance across different assessment instruments can be made for the same child.

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