As far as the Queen is concerned, the most important change in her lifetime is life expectancy. When Princess Elizabeth was born in 1926, the life expectancy at birth for a girl was 61.5 years and 57.6 years for a boy. Now it’s 82.8 and 79.1 years respectively and she has to send so many telegrams to congratulate people on their 100th birthday that her office has taken on extra staff in order to cope.

Of course, this has come against a backdrop of a large increase in the general population. At the 1921 census the total population of Britain was around 43 million compared to the 64 million recorded in 2011.

How do we define older “older people” and what is old age? Old age has a number of different conceptualisations. It can be defined as the last stage of an individual’s life, although we often only know that in retrospect. More usually we use a specific chronological age for policy and practice reasons to define eligibilities for benefits such as pensions at one end or to determine entitlements such as voting at the other. We also use specific ages to define specific segments of our population such as teenagers or older people.

The use of a specific age to define old age is highly arbitrary. In Britain, the 1875 Friendly Societies Act, defined old age as “any age after 50”. Conventionally though, most developed countries use the age of 60 or 65-plus to define the older population for demographic purposes. While there is some consensus internationally in the use of specific ages to define old age from a demographic or policy perspective there is no such agreement as to the chronological age at which individuals become “old”.

Actually, those life expectancy estimates we read at the top of this article are static – based upon the mortality rates applying at the time. If we use a cohort-based estimate (which includes known or anticipated improvements in mortality rates) then the life expectancies for babies born in 1926 are 70.5 and 64.2 years respectively compared with 92.2 and 90.4 years respectively for those born in 2014.

So as the Queen celebrates her 90th birthday, she lives in a country where, on average, all of her subjects being born now can expect to become nonagenarians.

Death deferred

We are seeing the emergence of older people as a distinct demographic group – there are now 15 million aged 60 or over (23% of the total population) compared with approximately 3.5 million people aged 60-plus in 1921.

What has brought about this demographic change? At the risk of stating the obvious, the key to the large increases in life expectancy are the decline in death (mortality) rates. This has come in a century that saw dramatic improvements in health care and social conditions. In 1926, the crude death rate was around 20 per 1,000 population compared with half this in contemporary Britain. Death is now almost exclusively confined to later life. In 1926, 15% of all deaths were of children under one year and a further 5% of those aged 1-4. Around 40% of deaths were of those aged 65 or more, compared with 85% of all deaths now. The decrease in death rates, especially in childhood and early and mid-life adulthood means that very few of us will die before we get old.

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However, decreases in death rates have been seen across the older age groups as well. This has resulted in the emergence of significant numbers of nonagenarians and centenarians. George V sent out the first congratulatory telegram to those who reached their 100th birthday in 1917. In 1921, there were 110 people included in the census who declared their age as 100 (or older), 70 of whom were women. In 2011, there were a little over half a million people aged 90 or over and almost 3,500 aged 100 (or older) in Britain.

Rise of the ‘super centenarians’

The next years of the Queen’s reign will see the emergence of the super centenarians – those aged 110 or more – as a distinct segment of the population.

It is estimated that one-third of children born in the Queen’s 90th year will live to be 100 or more. Of course this has the well-established caveat of “if current trends continue”.

We can see evidence of these changes in on our high streets where birthday cards for nonagenarians and centenarians are now readily available.

You’ve only got to look at Queen Elizabeth – and her husband Philip who turns 95 in June – to realise that being a nonagenarian does not imply inactivity or disengagement from society. We can see this in all sorts of different spheres of life. And it is perfectly illustrated by FINA – the world governing body for swimming – which now maintains world records for swimmers aged 100-104 and 105-109.

Christina Victor, Professor of Gerontology and Public Health, Brunel University London.

This article first appeared on The Conversation.