INTRODUCTION

Human biologists have devoted considerable effort to devising biological age measurement techniques for childhood, and these have been used with success in evaluating variation in developmental rates. Although the concept of biological age is equally appropriate to the ageing process, there has been much less progress in measurement of this latter phase of the life cycle. The lack of progress is in part due to the newness of gerontology as a scientific discipline, but also to the unique features of the ageing process itself. While the sexual, dental and skeletal development of the child progresses through a series of relatively invariant stages, ageing changes (with the exception of menopause in the female) are progressive rather than stepwise. Further, while most children are healthy, as adulthood progresses the divergence in health becomes considerable, and identifying completely healthy aged individuals is difficult. When the long duration of human ageing is recognised, and is added to the uncertainty about when it begins and ends, it becomes clearer why biological age assessment has been less successful for adults than for children.

Gerontologists have recognized the importance of measuring biological age in adulthood and a number of authors have discussed possible approaches to the problem (Benjamin, 1947; Comfort, 1969; Bourliere, 1970; Shock, 1977). Several investigators have developed biological age scales using diversified batteries of physiological tests. Typically, test parameters are selected based on their cross-sectional linear correlation with chronological age. This approach assumes that those traits which vary most closely with age are the best indicators of the ageing process.