[Copy-pasted from MuslimMatters.org]
The Qur'an mentions forty as the age of reaching full maturity: “Until, when (man) reaches his maturity (ashudd), and reaches forty years of age, he says, 'O My Lord! Allow me to thank the blessings that you have bestowed on me, and on my parents, and that I perform good deeds that are pleasing to you, and make my children righteous as well. Truly, I repent unto You, and are of those who submit totally to you” [Ahqāf; 15].
No wonder, then, that our Prophet Muhammad ṣallallāhu 'alayhi wa sallam (peace and blessings of Allāh be upon him) actually began receiving inspiration and preaching his message at the age of forty. For forty years, he was merely being prepared for the real purpose of his mission: the call to Allah.
This is the year that I reach that important milestone of life. I do not know what the future holds for me, although of course I have my visions and plans. But it seems fitting for me to pause and reflect upon the last four decades of my life, and ponder over its ups and downs.
I remember vividly many of my thoughts and emotions when I was twenty. It was exactly twenty years ago that I graduated from the University of Houston, and left for the Islamic University of Madinah, beginning a new phase of my life. I began thinking, “If I could, somehow, give my younger self some advice; if I could address the young man of twenty, now that I am forty, and hope that he would listen to my advice, what would I tell him?”
These are the top ten things that came to mind. I hope those of you who are still in their twenties (and perhaps some of you who are older!) will benefit from it.
1) Don't be so certain about your opinions and views.
Arrogance and cockiness define teenage years, and a young man (or woman) at twenty really is just a teenager, plus one. Views about how to live, about interpretations of religion, about how you would do things differently than everyone else in the world – those views typically stem from a naïve and inexperienced view of the world. You will realize that over-enthusiasm and strongly held opinions are the quintessential signs of being young. Don't judge others who disagree with your views too harshly: you just might find yourself holding those same views a few years or decades down the line!
2) The most important source of practical knowledge is life itself.
Continuing from the last point, realize that the single greatest source of wisdom is learnt by living life itself. No matter how many lectures you attend, or books you read, or how deeply you contemplate or think, nothing substitutes the wisdom gained from simply experiencing the world around you. In order to be a good spouse, you need to learn to navigate the ups and down of a marriage. In order to be a good parent, you need to have your own children and learn to take care of them throughout their stages of childhood. In order to be a good human, you need to experience the good and bad of humanity.
'Facts' from books are great, but they must be shaped and seasoned and tested on the playground of life. Appreciate that you might not be in the best shape to judge everything, especially since you might not have experienced those things before. Through experience, and trial and error, one's methods for dealing with all types of problems are refined.
A corollary of this piece of advice (and if I had more than ten in this list, this would be number eleven) is: Respect and benefit from those older than you. Perhaps you know more than an elder about a certain matter (or, to phrase it more precisely: perhaps you think you know more than them about a certain matter), but no matter how knowledgeable you are about quantum mechanics, or investigating sahih hadiths, or understanding the latest psychological theories from your textbooks, you simply cannot match the wisdom of your grandmother when it comes to navigating the intricacies of human interactions and raw emotions.