Well, not all above 40, turn spiritually. But why? Why not all?
God chooses only certain people who are ready mentally to seek God and live a life of high morality, extend unconditional love and live contentedly.
Usually, rather without exception, their life purpose is revealed to them by the divine forces.
What I imply is, there is absolutely no need to discover your life purpose by reading books, talking to sages and saints etc.
If God sees you are ready, then, he will show you what you have to become.
A life purpose, in my opinion, is revealed to you or you will be made to realize when you pursue your passions and are truly willing to share whatever you know and whatever you can give.