(1) Random rules like minimum age for entering school can advantage some children at the expense of others. These children who are not very successful to start with come under the positive effect of "Self-Fulfilling Prophecies" which evoke the behavior of success.
(2) Those who are already successful are more likely to get opportunities that lead to more success. This is "Accumulative Advantage" at work.
(3) Ability, opportunity and arbitrary advantages combine to create success. If you want to master anything just spend 10,000 quality hours at it and you will be world class. This applies to whatever you want to do-Maths, Literature, Business, Sport, Music and even Robbery!
(4) Extra-ordinary talent is shaped by extra-ordinary opportunities. Be thankful to God and be humble if you are successful because you may have been given extra-ordinary opportunities which may have passed others due to some very random reasons. To make matters more complicated; the world at your time should be able to appreciate your 10,000 hours and reward you for it!
(5) IQ is not everything. Creative Intelligence (divergent logic) is much more important for success than Analytical Intelligence (convergent logic). Practical Intelligence means being able to handle ground situations well. Unlike Analytical Intelligence it is not a genetically determined trait. It is a result of "Concerted Cultivation" of the child's personality by the parents in which the child is taught independence and is exposed to decision-making almost as an adult would.
(6) Sometimes some people are unsuccessful just because of "Demographic Reasons". Nothing seems to work in spite of their intelligence and hard work because social and economic structures are unfavorable to different professions and different classes at times. So we must learn to read the signs of the times to escape this limitation.
(7) Hard work is a prison sentence if it does not have meaning. To have meaning in your work; you must have some "Autonomy" while working, a reasonable amount of "Complexity" to handle and there must be a clear connection between "Effort And Reward". Successful people endure rough conditions and low payback if their work is meaningful and this endurance leads to breakthroughs irrespective of humble origins.
(8) Cultural legacy is a strangely powerful force which can work for you or against you. You need to break out from your cultural legacy and re-train yourself if this stands in your way. Its obvious at times that your skills just don't matter if cultural legacy is standing in the way. Cultures that place a high emphasis on hard work are likely to produce more winners in almost any and every related or unrelated field.
(9) Finally success is not mysterious at all. It is a product of History, Community, Opportunity and Legacy besides Intelligent Hard Work. This thought keeps us from the pride of our achievements.