Good question!
Developmental Psychologists have long used either ages or stages to define segments of the population. Many developmental textbooks are divided into chapters based on one or the other of these definitions.
Age definitions are ones that use numbers – such as: being 65 or older means you are a senior citizen; or all over 55 get a senior citizen discount.
Stage definitions are usually feeling and behavior related – along the lines of: “you are only as old as you feel [or act.]”
But over the last few decades, as we learn more and more about the later stages/ages of the lifespan, the thinking is changing – and so are the textbooks and definitions.
Why?
Because the field of Developmental Psychology is itself aging – as are the original developmental psychologists – and more information is becoming known and understood about life’s processes.
And add to this that we are living longer. In 1940, the average life expectancy at birth in the USA was 62.9 years; in 1960 it was 69.7; by 1980 it was 74.1 and in 2000 it was 77.2. [The statistics differ by sex and ethnicity but these are the averages for all persons.]
Average only is a middle figure. Half die before and half after the ages cited. And if one lives past infancy, life expectancy increases and it increases every year one is still alive. So those who were born in 1940, and are obviously well past 62.9, have a far different life expectancy than when they were born. That expectancy is now somewhere into their mid 80s.
So as to what makes you a senior citizen? It’s up to the language we use, the current psychology and of course, legal definitions.
Most jurisdictions rely on when you can start collecting social security benefits to define what is their senior population. But that has been changing too and if you were born in 1960 or later, you will not be eligible for full Social Security benefits until age 67. But as we can still sign up for Medicare at age 65 – 65 seems still to be the “age” definition of senior citizen.
Will that change? It might…but not for those who are already at or near 65. We ARE labeled senior citizens.
And what about behavioral definitions – the stages aspect?
That is up to us. We can continue to do what we have been doing – living life to it’s fullest and not becoming the stereotypes many have of what a senior citizen is.
We are who we are – and are the ages we have accumulated!
If we let someone else’s characterizations of “senior-ness” define us or our behavior – then we are falling prey to stereotypes.
I am of the thought that we are only as old as we feel and act!
So feel and act young!
You may still be a “senior” but you’ll wind up confounding a lot of people 
This article can also be read at:
I personally do not believe in the concept of anti-aging because we cannot really go backwards or stop our aging process. 
and I think at 70 – I still look great]
[I post this poem every year]