There are societal norms I've never understood. Our culture trains us, sometimes brainwashes us, into believing there is a way to think about ourselves and you conform to that or suffer being called "a freak."
The only time in my life where being a certain age bothered me was when I turned 30. After that, a birthday was a birthday. I love celebrating my birthday. Who doesnt like a day when it's all about you? Some might argue that I think it's always about me but that's another story.
Why is being middle-aged believed to be something to have a crisis about? By modern statistics, if you live a half-way healthy life you will live another forty years - another lifetime to where you are now. Hitting forty gives you a chance to review and see where you have been, where you are going and decide if you want to keep going in that direction. Most night-time college classes are full of middle-aged people who have decided they another professional direction. You do not have to choose one career in this life and only adhere to that path.
What about a personal direction? I think the pressure to be in situations were are not happy with keeps people where they are. For all of our advertisements about personal choice and individualism there is precious little of that in actual practice. What will so and so think? Am I cool? If you ask yourself either question you aren't doing what you want but what will please others. There is far too much of that.
We are a judgmental society and though there are many more freedoms today for minorities and other viewpoints and lifestyles than there was even ten years ago, the personal freedom of doing what's right for us is still blocked by a concern of pleasing others and measuring where we are in our professional and personal lives against those who follow what we believe to be "the norm."
Hence the perceived middle-aged crisis.
We don't value having learned key lessons in life and being comfortable in our skin. That's not to say that continued growth and learning doesn't happen any more; you're dead where you stand if you stop exploring yourself and the world around you. Relationships are shot to shit because once in a relationship, it's like vacation time when the real work is only beginning. You can find your soul-mate and things still not work if you forget that the soul needs to be re-energized and nutured as any growing entity does.
None of the above ever stops. Change is a natural and necessary part of life. A look at what is working for you and what's not is never a crisis no matter how old you are. There is a tendency to do this at the age of around 40 but the answer isn't any different at 40 than at 30 or 20.
There's no crisis. Only hope and the excitment of new horizons and challenges.