Accumulating data on cognitive functioning in later life are yielding a clearer understanding of the complex patterns of change. The Seattle Longitudinal Study (e.g., Schaie 1983, 1994, 1996) provides typical findings. Cross-sectional data from 5,000 adults from age 25 to 88 show consistent negative age effects on such latent abilities as inductive reasoning, spatial orientation, perceptual speed, and verbal memory (new learning). In contrast, numeric ability (simple arithmetic calculations) and verbal ability (synonyms and recognition tests of meaning) show improvement until midlife and then a plateau until the oldest tested age of 81. People at age 81 performed at a higher level on vocabulary tests than people at age 25. These data are for both speeded and unspeeded vocabulary tests combined. If only speeded vocabulary tests are considered, performance declines start in the 60s, although the rate of decline is still lower than for other cognitive functions. Longitudinal data in the Seattle study show an inverted U-shaped age function for most verbal and numerical abilities, with the highest scores achieved at ages from the 30s to the 60s, depending on the ability. World knowledge is usually found to be stable into old age, for example, as measured by the WAIS information subtest (Salthouse, 1982) or general information questions (Camp, 1989; Nyberg et al., 1996; Small et al., 1995; but see Hultsch et al., 1998).
Schaie's data are from psychometric tests that were not designed to identify mental processes and mechanisms underlying the cognitive functions being measured. Using laboratory techniques from cognitive psychology to investigate semantic memory processes, studies consistently report age invariance in semantic organization and processes, and in a variety of other language processes (see Kemper, 1992; Light, 1991; Burke, 1997). This extends even to discourse-level processes, because consistently higher ratings are given to older compared with younger adults' narratives (e.g., James et al., 1998; Kemper et al., 1990). Recent findings, however, demonstrate that some processes involved in language production decline in old age: older adults suffer more failures in retrieval of phonology (word finding failures) and orthography (spelling errors) than young adults, despite their superior vocabularies (Burke et al., 1991; MacKay and Abrams, 1998). This pattern of dissociation in age effects on language functions is at odds with descriptions of age invariance in crystallized or pragmatic functions. Together with dissociations in age effects in other cognitive domains (e.g., explicit versus implicit memory), these data pose a challenge to models of cognitive aging.
Cognitive functions are not only a matter of the speed and accuracy of information processing. The aging mind also involves cognitive contents, such as ideas of self and the meaning of life, and skills that go beyond speed