But, I don't think it's fair to say it has to come from inside the person somewhere. Obviously, you're right in the literal sense, as you can't impose delayed gratification on a free adult. But, that doesn't mean its inherent in someone's genetics. I think the far more likely reality is that people learn delayed gratification by watching those around them. Even if you do have some preternatural foresight as a child, it's extremely difficult to see how delayed gratification makes you successful in life if you never see anyone that's successful due to their practice of delaying gratification.
This cuts across wide social divides as well. I grew up in a wealthy family, in a wealthy suburb and had wealthy friends. Everyone had wildly successful parents. Guess which kids ended up losers (and there are a lot of them)? The ones whose parents were too busy with their careers and placated their guilt by trying to buy their children's happiness. These were the kids who got everything they wanted, any time. These were also the first kids to smoke, try drugs, have sex, and play video games instead of do homework or go to football practice. Now these people are working minimum wage jobs, if they're working at all. The few who used their family connections to get into sales gigs, or similar, are generally broke. They make a little bit of money, then blow it all in a weekend.
I can only imagine how this effect is amplified growing up in the lower socio-economic classes. My friends at least knew people that became successful, and saw that it was possible. I'm sure it's easy to think that improving your life is impossible if you'd never met anyone who did it, or if your parents, family, and friends were impoverished, or alcoholics, or drug addicts.