How does DNA get damaged? What causes such vast differences among people? Some are born greater than and others will always be less. Why?

Here's what me thinks ... 

This is the old nature vs nurture discussion. 
Despite what is necessarily fair under law, no matter what we might wish to be true, nature has shown with great frequency and display that all are not created equal. Period. 

People differ in their many qualities because we are each products of a unique set of circumstances. We each have our physical features, size, eye and hair color, etc, from the unique set of genetic instructions passed to us from our parents, and their parents, so on and so forth. Mutation, by the imperfect replication of, or unique intermix of genes determines much of this. 

How our minds are individually 'wired' also is a product of genetic inheritance. We see things differently because our perceptions are not consistent. We don't all process thoughts in the same way. Just as some people are larger and stronger than others, some minds are inherently more capable or agile than others. 

If we add or (more accurately) compound the natural, innate attributes of our personality and aptitudes (or specific lack thereof) with the environment, beliefs, advantages and opportunities specific to one's upbringing, a base of understanding, abilities, and perception emerges that is unique to each individual. 

How the individual adapts his own gifts, abilities or deficiencies to life and the world then becomes subject to the sum of their decisions and consequences imposed by the random nature of life and the world. This post was edited by Benedict Arnold at May 21, 2017 10:35 AM MDT

Intelligence is innate, knowledge is learned. 

Damaged DNA is an entirely different issue, it's becoming apparent that autism may be caused by a damaged X-chromosome ... and this is affected by carcinogenic agents which act upon the maternal grandmother, which may be why there has been a spike in it.
A man produces sperm all his adult life, but a girl is born with all the ova she will ever have - and if any of these are damaged while she is in utero, children conceived from same are more likely to suffer congenital defects. If a woman smokes during pregnancy, her daughter may be born with damaged ova - so her grandchildren are at risk. There are other carcinogenic factors of course - particulates from diesel and coal fumes, radioactivity and so forth - but smoking is crucial, prior to the Second World War "nice" girls didn't do that. Then the cigarette manufacturers got stuck into Hollywood, and all the glamorous actresses started lighting up on screen. Voila - a generation of nicotine-addicted mothers! The insidious nature of this did not become apparent for twenty years or more - when their daughters started giving birth to disabled babies.

Slarti and Z did a pretty good job here, but I'll add that Mother Nature doesn't really have a plan. She seeks to create diversity in a million different ways. And, the traits that are most beneficial get passed on more often, simply because people with them have more opportunity to reproduce. Some of the most brilliant problem-solvers in history- mathematicians and scientists- were likely autistic. Some would call that a disability, and perhaps it is, but the presence of these individuals and the way they process things differently, has changed history. According to one article that I grabbed quickly from Google, Mozart, Einstein, Newton, Darwin, Michelangelo, and Thomas Jefferson were all likely autistic. Imagine a world that they never existed in.

I don't know what you're trying to get to the heart of here, but a low IQ isn't always the result of damaged DNA either. Sure, damage plays a role, but we're designed to be different. Those with naturally low IQs probably just have them because their parents did. Maybe that trait got passed on because the ancestors were incredibly beautiful or had another trait that made them attractive to mates. Again, nature doesn't have a plan.

its just the way god made us, not sure why