In gerontology, there are two types of ageing.

  • Normal ageing (primary)
    We define normal ageing as the biological process that involves functional or physiological decline over time, and this is in proportionate with an increased risk of disease and death.
  • Pathological or premature ageing (secondary)
    Pathological aging is a condition of losing functionality independent of ageing itself. For example, preclinical Alzheimer’s disease.

We can classify age as:

  • Biological (physiological) age
    the age using lifestyle factors such as fitness, skin elasticity, greying hair, sleeping habits, and sexual maturity as the determinant.
  • Chronological age
    the measurement of the years that an organism lives.
  • Psychological age
    the maturity and ability to cope with circumstances, self-awareness, problem-solving, etc.
  • Social age 
    defined using a scale measuring the interpersonal skills and ability to fulfil the norms and expectations associated with particular social roles.
  • Functional age
    the physical and cognitive performance abilities, it represents a combination of the physiological, psychological, and social age.

In recent decades, there is a scientific definition of age called molecular or cellular age. At the end of our DNA, there is a protective cap called telomere; in every cell division cycle where DNA replicates, the telomere gets shortened; after cell division, the cap folds back to protect the DNA. Once it gets significantly short, it can trigger a loss of DNA repair function, loss of cellular regeneration function, subsequently lead to cell death or potentially mutation and cancer. Theoretically, every time a cell propagates is a cycle of ageing, but this age is not comparable between two individuals (except twins), simply because every individual is unique, and their starting telomere length could be different.