The Three Ageing Curves and The Disability Threshold
We all age differently. The question is not whether we age, but which ageing curve we are following now, and which curve we seek to follow in the future.
If you’ve undertaken my quiz, you will have an idea which curve best suits your current lifestyle. The question you need to consider is which curve do you want to pursue moving forward.
Research identifies three potential life curves that demonstrate how physical and mental capacity changes over time. These curves help explain why some people remain independent and active well into later life, while others experience earlier decline.
The Three Ageing Curves
The human body begins to age at 35. These three ageing curves - typical, optimistic and successful - represent the manner in which we age.
The three ageing curves, in reverse order, from the most common to the least common, are described below:
3. The Typical/Usual Curve
The Typical or Usual Life Curve shows a steep decline in the ageing process. It has often been described as resembling a steep ski slope. On the Typical Curve, people start taking medication in their mid-50s, and experience ill-health in their mid-60s onwards.
People following this curve are usually sedentary and have a number of behavioural risk factors – e.g. smoking, poor diet, and lack of exercise.
They experience earlier and faster loss of strength, function and resilience than is necessary. This pattern represents approximately 60% of the population, making it the most common ageing trajectory in Australia.
2. The Optimistic Curve
The Optimistic Life Curve shows a lesser decline.
Individuals on this curve generally have a more optimistic approach to lifestyle and make some positive health choices. While decline still occurs, it happens more gradually than on the usual life curve. On the Optimistic Ageing Curve, people usually start taking medication in their mid-60s, and experience ill-health in their mid-70s onwards. Approximately 20-30% of Australians are pursuing this ageing curve.
1. The Successful Ageing Curve
Individuals following the Successful Ageing Curve strive to maintain, as closely as possible, the physical and mental attributes of a 35-year-old until the end of life – where you ‘drop off life’s cliff’.
Clearly, some decline is inevitable. However, this model also includes the concept of compressed morbidity, where the period of illness or functional decline is shortened and occurs late in life rather than gradually over decades.
People on the Successful Ageing Curve pursue a healthy lifestyle with plenty of exercise, a good diet and avoid other risk factors. On the Successful Ageing Curve, people may only need medication to combat ill-health from age 85 or beyond, and enjoy a greater quality of life as they age. Only approximately 10-20% of Australians are committed to this ageing curve.
This website encourages you to pursue a Successful Ageing Curve and live the dream.
Remember: life is not a dress rehearsal.
The Disability Threshold
The disability threshold is a level of decline when people can no longer care for themselves. The threshold for each ageing curve is shown cross-hatched in the diagram above.
The hypothetical disability threshold is reached when a person can no longer look after their own daily needs and requires assistance to undertake normal functions.
If someone is pursuing the Typical Ageing Curve, there is a strong chance they will fall below the disability threshold, as shown in the diagram. While this level of decline can often result from disease or cancer, it may also be the result of usual ageing and exposure to a number of risk factors. On the Typical Ageing Curve, decline occurs much earlier - usually in the mid to late 60s - and people require assistance for longer.
On the Optimistic Ageing Curve, decline falls between the other two ageing curves - usually in the late 80s - and there is less time spent where is assistance is required.
On the Successful Ageing Curve, the disability threshold is reached much later in life - usually in the 90s, and there is less time spent in decline.
Why the Disability Threshold matters
As we have seen, usual ageing is frequently the result of exposure to behavioural and biomedical risk factors. These can weaken immune response and increase vulnerability to infection, illness and functional decline.
One of the key problems with ageing is that many people believe disability and decrepitude are inevitable. This belief itself becomes a powerful risk factor.
An observation by Marilyn Ferguson is particularly relevant:
“Of all the self-fulfilling prophecies in our culture, the assumption that ageing means decline and poor health is probably the deadliest.”
By understanding these curves, and recognising the role lifestyle plays in determining which path we follow, we can begin to make informed choices about how we age.