The American Heritage Dictionary of Idioms cites the facts of life as:
Knowledge of sexual reproduction, as in Some people feel that the facts of life should not be taught in school. [Late 1800s] Also see birds and the bees. [http://www.answers.com/topic/the-facts-of-life]
Aging Process And Death
The facts of life include an unstoppable aging process until we finally die. No person is exempt from dying regardless how he lives, exercises or eats. We are all going to face death and leave everything behind. In fact, this cursed age even causes the trees and animals to die too. Such is reality.
Spiritual Facts of Life
Furthermore, our earthly life is a series of spiritual tests from God (Prov. 17:3) as we face trouble every day (Mt. 6:34) and reside in a truth rejecting world where our enemy, the devil, freely roams about to tempt people to sin! Our short lives are filled with temporal goals, cares and pursuits which can overshadow the will of God. Wise is the person who doesn’t allow that to happen, but instead puts into practice God’s word (Mt. 7:24-27). Are you wise in that way?
Why, you do not even know what will happen tomorrow. What is your life? You are a mist that appears for a little while and then vanishes. (James 4:14)