This happens all the time whenever there is a "craze". When marijuana companies were the craze, scammers who peddle stocks that sounded similar to high flying stocks.
Also, the more wealthy you are, the better you'd be at delaying gratification, even if you are very young. If you never know hunger or want and you know that the "marshmellow" will always be there, then you can more easily delay gratification. But if you are poor and you know that the marshmellow might not be there tomorrow, you are less likely to delay gratification.
Not just fluid intelligence, but overall energy, enthusiasm, etc.
A lot of old people here seem to be clutching onto "experience" but it is heavily overrated in a society/industry based on learning new things.
Younger people have more potential ( intellectually and physically ), they can work more and they are able to learn new things far more quickly than old people. Add to that the fact that young people are already immersed in new technology and they fit more naturally in a younger tech work force.
It's like asking why magazines seek younger men and women? Because it works and it's the optimal solution.