A middle-aged adult is anyone who is 40 to 65 years old , and it is a time in life with its own unique set of challenges; a middle-aged adult has their youth behind them, yet their future is still in front of them, but death is now on the horizon, and this often results in a midlife crisis. If you, or someone you know, is a middle-aged adult, then I have good advice for you, from Karl Barth. Karl Barth's assistant and biographer, Eberhard Busch, scoured Karl Barth's life works, consisting of myriads of pages and millions and millions of words, and selected about 100 one-page long, practice words of wisdom, and compiled them into a very easy to read book called Insights: Karl Barth's Reflections on the Life of Faith. And one of these insights is for middle-aged adults, that I found particularly helpful.
When I became an adult, I put an end to childish ways. (1 Cor 13:11)
The middle years of life offer a very special opportunity to be mature. It could be that the eye of the no longer young and not yet old person is especially free from the fog that, for the young person, can still veil the urgency of decision--and from the shadows that can do the same for an old person. Those in their middle years can come to a sudden realization: "It's now or never!" The sowing lies way in the past; now it's time for the harvest. The run-up has been done; now it's time to jump. the preparations have been made; now only the venture of the work itself actually remains. They have already lived, and they can still live. They now have a still considerable past and thus experience, but that should not yet have made them tired and stiff. Also they already see the end from some distance away and know that "night is coming when no one can work" (John 9:4). They see it, however, still at such a distance that the idea tempts them neither to resignation nor to last-minute dash; rather, it should only urge them on with all deliberate speed. Thus they could also be especially free for the now and for the commandment of God that claims them in their now, both retrospectively and in anticipation of the future. Their position in the middle of life, between the times, has the character of both broadening and gathering. will they recognize and seize their opportunity? 
This insight is a good example of the types of insights contained in this book, and I believe this is a great book to use in a discussion group, community group, small group, because this book is easy to read and anyone can understand it because it is not laden in theological jargon. So this book is especially good for people who have never read a theology book, or do not know how to pronounce "Barth".
1. Wikipedia: https://en.wikipedia.org/wiki/Middle_age#Definitions
2. Karl Barth, Insights: Karl Barth's Reflections on the Life of Faith, ed. Eberhard Busch, trans. O. C. Dean Jr. Westminster John Knox Press, Louisville, KY. 2009. p. 45 (Selection from the Church Dogmatics III/4, p. 705-6)
Related: Eberhard Busch, Insights, Insights: Karl Barth's Reflections on the Life of Faith, Karl Barth, middle-age, middle-aged, midlife crisis, O. C. Dean Jr.