There is, of course, no strict numerical and chronological division of life - but if we are talking about an approximately threescore and ten year lifespan (and anything beyond merely an extension of old age); then we could reasonably follow CG Jung and divide it into quarters; then we get four periods of about 18 years each.
This would mean that old age began at about 54 years old.
That seems about right to me.
This four-part division is - of course - somewhat circularly-defined; in the sense that it is partly based on observation and partly based on what ought to be. In other words, childhood and development not only do, but ought to continue to about 18-21 (childhood lasts longer for men than women) - and it would not be a good thing for it to end much earlier or much later than that.
Perhaps the least obvious is the idea that adulthood is divided at about 36 years old (between something like 'young adult' and 'middle age') - and this can only easily be seen among creative people such as artists, authors and research scientists, because mid-thirties is about the time when there is a transition between learning the craft or profession and beginning to contribute in an original way.
It is the division between master-ing and being a Master.
Even among 'precocious' creative people, who begin to make contributions in their twenties; the very best work usually comes between the middle thirties and middle fifties. For instance, Einstein made major scientific contributions in his middle twenties; but General Relativity was the achievement of his middle thirties.
With respect to old age; the point is not merely that there is a decline in the quantity and quality of publicly recognised attainment in most people; but that there ought to be a change in emphasis away from extraversion to introversion; from this-worldly achievement towards next-worldly orientation.
A shift from duty to conscience.
And this 'ought' applies to the individual, and has a cosmic and spiritual aspect.
In modern materialistic society, then the external perspective is the only one that is regarded as really-real; so old age is merely a time of declining productivity on the objective public side, and the struggle to delay (or at least deny) this decline on the subjective private side.
The notion that old age is - in some respects - like childhood, has considerable validity. Both are periods of mostly-dependency, in a materialist sense. Both are less worldly-active and more 'contemplative'.
Both childhood and old age should also be times when the self and the world are less sharply divided, and the world is more 'animistic', more a matter of beings than things... The night is more important, dreams are more important, the spirit (and religion) is more important.
The not-here and not-now, the not-worldly/ un-worldly looms relatively larger.
There is, however, a major difference in consciousness between childhood and old age.
The child is 'in' the world (albeit this dwindles through childhood), takes it for granted and has never been otherwise; but the old person has stood-outside the world, has worked upon the world - has grappled with the present moment - and then moved away from the world and the moment. The longer he lives, the more the old man's consciousness moves away-from the world.
In old age, memories of childhood take on a greater sharpness and spiritual power; in many respects childhood is re-experienced. But these are memories, so the experience is consciously known as such, when the child simply lived it.
The matter can be focused by a single word: death.
As soon as a child is fully conscious, he becomes aware of his own death as a pervasive possibility. All children spontaneously have theories of death and may develop rituals about death. Then death recedes from adult consciousness as the world grows.
Through old age, death returns to awareness: 'the dead' loom ever larger.
Old is age is 'about' death - the last phase of life is structured-by death; and therefore the quality of old age depends on how death is understood.
For the modern materialist humanist, this means 'coming to terms with' imminent extinction - trying to accept, or even desire, extinction.
For the Romantic Christian, however, 'coming to terms' with death means something very different; because death is not the end; but instead the necessary gateway to a positive transformation.
The task of old age may be learning about the nature of this transition, its possibilities, and how to 'manage' this transition.