The mind of a child has a much simpler thought process.When you expose them to a test, it is less likely for them to think too much into it where adults might even try to break or game the test. Testing someone at a your age can result in actions that closely resemble the inborn instinct of an individual. Stanford marshmallow experiment is one such experiment.

A 2018 study by Tyler W. Watts, Greg J. Duncan and Haonan Quan revealed that by the time the kids reach age 15, the marshmallow experiment becomes less useful.

We replicated and extended Shoda, Mischel, and Peake’s (1990) famous marshmallow study, which showed strong bivariate correlations between a child’s ability to delay gratification just before entering school and both adolescent achievement and socioemotional behaviors. Concentrating on children whose mothers had not completed college, we found that an additional minute waited at age 4 predicted a gain of approximately one tenth of a standard deviation in achievement at age 15. But this bivariate correlation was only half the size of those reported in the original studies and was reduced by two thirds in the presence of controls for family background, early cognitive ability, and the home environment. Most of the variation in adolescent achievement came from being able to wait at least 20 s. Associations between delay time and measures of behavioral outcomes at age 15 were much smaller and rarely statistically significant.

Full Research Paper At: http://journals.sagepub.com/doi/10.1177/0956797618761661

Why?

Individuals grow and become more capable. Walking 2 miles could be a big deal for a 4 year old. But even 5 miles isn't that huge for an adult. A dollar is barely worth anything for someone living in a first world country. But for someone in Venezuela, few dollars is all they earn in a whole month if they are lucky. Maybe the marshmallow was a little too little for the teenagers. Maybe an iPhone would have done the trick. Maybe the waiting time was too small. People become more patient and disciplined when they grow, so 15 minutes could be something small for them. Maybe the teenagers have been deceived in their life and they are less likely to expect people to keep their promises.

But one thing is clear. It's easier to measure and influence a very young child.

The Economic Implications

@jaichai actually made a great post titled Pillars Or Pariahs: Is Your Diet, Daily Routine And Mindset Helping or Hindering Your Willpower? which you should all check out. There were lots of great content in the post.

It is common Austrian wisdom that to really build wealth you need to consume less than you produce and invest the saved difference to create a positive sum game. If I pick an easier to relate example, I'd say look at STEEM blockchain. Like any other social media, you'll have to wait and build you followers and portfolio of content. Even if you are one of the greatest writers on the planet, you won't get that much recognition at the beginning. An year ago I was a tiny plankton with a tiny reputation earning barely nothing. Now I'm managing to earn few dollars everyday as a result of my hard work. I've manage to rank among the 1% under many metrics and when it comes to reputation I'm sort of on the Top 500. That's after a whole year on the blockchain.

Some people have done way better. But even they had to put some serious effort. Then they had to wait. If you have done any trading, you'll know that you have to play a very long and patient game. Personally I think learning to become a good trader can give more discipline to a person in a very efficient way far better than any military ever could.