Studying change over a lifetime poses obvious logistical problems, which one group of researchers sought to overcome by gathering data retrospectively on famous people. In their study, published in the Journal of Personality and Social Psychology, they looked at all the written material they could find by authors such as William Shakespeare, Louisa May Alcott and Charles Dickens at various times in their lives. Then they analysed how writers' perspectives on life altered by the language they used both in their works of fiction and in their personal journals, papers and letters.

They discovered, as expected, that older people use more complex language and longer words, indicating more complex cognitive thinking. But they were surprised to find that older people tend to talk about the future considerably more than younger people do.

Another big contrast was in how often these writers mentioned other people. From the age of 25, it seems, individuals talk (and write) more in generalities. The most dramatic finding in the private writing was the increase with age in the use of positive language.

The researchers saw these changes as the result of increasing self-reliance and independence. Under-25s may depend more on other individuals in defining their evolving sense of self.

Shakespeare was 34 when he wrote As You Like It, in which the character Jacques describes the seven ages of man; these focus on the roles we play throughout our life, though these, of course, relate to character change. In Shakespeare's time, life stages were compressed by today's standards. He left school at 13, started a family at 18 and died, after a five-year retirement, at 52. Research finds it most useful today to divide adulthood into three stages: youth, middle age and old age.

Between adolescence and about 30, according to research in the US by the National Institute of Ageing, there are marked changes in our personalities on all levels that can be measured in standard personality questionnaires. The same trend continues after 30, but the pace slows dramatically. People become less anxious and irritable, and less reserved. They also start to be more conventional and practical.

Similar research has been carried out in Europe and Asia, with similar results, showing that even with a different set of social and cultural pressures, adolescents pass through very similar personality transformations. Younger people are more open to new ideas and are more enthusiastic. By 30, it seems, most have lost these qualities and attained some degree of maturity. Yet, they retain youthful cognitive flexibility. As the years roll by, perception and certain kinds of memory skills wane. Age has compensations, however. As noted, older people are less selfish and more patient, better organised and better adjusted. And, crucially, they are more experienced.

Nevertheless, ageism persists, with some unexpected implications. Although they may not be aware of it, people pick up on negative perceptions of age and the effect can cut up to 7? years off their lifespan, according to a study by Yale University. 'The encouraging message is that positive self-perceptions conveyed by society can prolong life expectancy,' the authors told Monitor on Psychology.