What is central to growing up?
Constant personal growth.
How so?
There is a phrase in the Chumash that says ‘Avraham was old and he came with his days’. Rabbi Samson Raphael Hirsch explains this to mean that every phase of life has a lesson, and this means he came with his days – he took all the lessons of every phase of his life with him going forward. Each stage of life has its challenges, its advantages and disadvantages, different opportunities and difficulties, and different lessons to be learned – and we need to appreciate and build on previous experiences and incorporate the lessons into our life.
Learning from the past and looking toward the future?
Life is what is happening now, and we cannot wish it away. So often people think ‘I just need to get through this and then everything will be fine’. Or they feel wistful for a time gone by. So either they are living in the past or the future. But each phase of life is given to us because Hashem wants us to be there, so that we can use the lessons we have learnt already to respond in the appropriate way.
But what about being unique?
These lessons are universal only in the sense that experience is. Everyone has a unique set of circumstances, given to us by Hashem, unique and crafted especially for us. We have to do our best to work with those circumstances. We see this in the Ramchal’s classic work Messilas Yesharim (Path of the Just) where he says, “What is a person’s duty in ‘his world’?” Because everyone has their own world which is unique to them. No two people are the same.
How does one graduate to the next phase of life then – merely by age, or by experience?
One has to be careful not to imply that everything is cut and dry, and each phase of life is like going from A to B. One’s life changes from day to day, month to month year to year, and there are constant changes even within that. What we need is to be alert to the fact that life and circumstances are full of constant change. And we have to adapt and respond to the uniqueness of each moment. 
Why then is change so hard to deal with?
People crave stability and familiarity, and this is part of the human makeup. But at the same time, it is also a part of us to crave newness. Human beings are caught between these two seemingly opposing cravings, contained in the phrase “Renew our days as of old”. Judaism teaches us that we need a balance between the constants and the ever-changing.
What are the constants of which you speak?
They are the eternal, immutable principles of the Torah. That doesn’t change. But the circumstances of our lives do, and sometimes this will affect how we are able to carry it out. In the Mesillat Yesharim the Ramchal says that everything in life is a test. Not only the different stages, but also the circumstances. He says that wealth is a test, and poverty is a test, health is a test, and sickness is a test. Wealth and health can test one’s arrogance, and gratitude to Hashem. Poverty and sickness tests a person’s ability to connect to Hashem in moments of suffering.
Where can we find the tools to pass these tests?
All the tools we need in life to grow and develop in the right direction can be found within the teachings of the written and the oral Torah.  As the Mishna in Pirkei Avot says : “Turn it over and over  for everything is in it”.
But if we make the ‘constant’ apply individually, does that not imply changing it?
We don’t ever change the Torah. But within it, there is much room for expression of the individuality of each person. Each person has to be constantly alert to the fact that Hashem has placed him or her in this moment right now because there are unique things that they can do, unique messages that they can learn, and unique challenges to help them grow. Every set of circumstances is sent by Hashem because we need to do something within them, and grow and learn from them right up until our last breath.
This differs from societal constructs, which values youth above all else…
Yes. That is why we have a mitzvah to stand up for an elderly person. Because look how much experience they have gained through life. Wisdom comes with age. And we look up to that, because one of the great values within Judaism is wisdom, and the older a person is, the more wisdom they should be acquiring. But there is another big difference between the way society views age and the way we do.
And that is…
The concept of retirement. Society’s view is that work is only work that is financially remunerated. If a person is not working for money, then they are not doing anything valuable. But retirement is only the end of economic productivity – not all productivity. The mission of being alive is to grow constantly. A person who has reached old age has to continue to develop as a person growing closer to Hashem. Those duties never cease.
So old-age can be seen as an opportunity …
Yes. Obviously one has to have the physical wellbeing to be able to do it, but old-age affords opportunities, such as more free time, which would allow for greater development in all areas of Judaism.  For example, one area in particular is that of learning Torah, which is so dependent on time available.  There are many opportunities to learn whether through shiurim, tapes, books etc.Using your potential…
The term for a human being is ‘Adam’, ‘from the earth’. The Maharal asks why a human being is named after the earth, especially since the Divine soul within each one of us is the principle part of who we are.  He answers that the earth is pure potential, and so is a human being.  Whatever you do with a piece of land will determine whether it achieves great things or not.   If the land is ploughed, and looked after, and watered it can produce great crops.  And so too the human being is pure potential.  It depends what we make of ourselves. And life is the process of actualising potential. While a person is doing this, they are alive as a human being, because this is what Hashem put us in the world to do. That is why the Talmud says the wicked are actually dead even when they are alive, and the righteous are alive even when they are dead.
But can we never just ‘be’?
All the age-old clichés – I am what I am, a leopard cannot change his spots, you can’t teach an old dog new tricks – are not so. We need to learn new tricks, stay fresh, and stay interested and interesting – that is what life is all about. There are two dangers to this. A person ceasing to grow, to say ‘I have done what I have done’ and then to rest on his laurels. Or he says ‘I am too busy now, I’ll get to it when I have time’. Pirkei Avot says ‘do not say in my leisure I will learn Torah, because one may not have the leisure’.
The danger of procrastinating? Nothing gets done.  Judaism is about living wisdom.  It is about action and making a real difference to the world, and to our lives and to the lives of those people around us.
So we have to live in the moment…
Life is dynamic. We need to seize the day, no matter what day it is, how old or young one is, or what the circumstance. This is why Judaism doesn’t see life in clearly defined stages. There is a commonality in every stage – and that is growth and improvement.