I think Yahoo, or a bigger company like Google would more likely pay very close attention to whatever it was that the kid invented, and offer to buy it from them at a price that is insultingly low ball, (unbeknownst to the inventor) that is far cheaper than what it is actually worth, while working diligently in the background to come up with something much better on their own after having figured out how it works.
Once this happens, they would then sell their version, for much less than the kid ever could, because they are bigger, and have more money, and can reach a larger number of people, therefore undercutting their profits, if the kid initially refused to sell it to them.. Or, if the kid did agree to sell, then they would not use the one they bought from the kid, and keep it on the shelf while siphoning ideas from it, and use it to make their version better, to sell it at an even higher price.
The kid at this point would be pretty much helpless, and would not be able to sue, or come up with something better, most likely due to being confined to the terms of the contract they signed when the transactions took place. If they tried to sell another version to another company, then the kid would be sued for breach of contract, (that they likely didn't read amongst the excitement and distraction from the [seemingly] large amount of money they were offered for selling their invention) and the company they sold it to would be sued for copyright infringement, that is, if they ever did show interest in the inventor after the original was sold.
Unfortunately, that is the way most big businesses work, they thrive on the idea that "everyone has a price," and "if you can't beat them, buy them." It is why today, creativity doesn't get you very far unless you are business savvy as well, and it is why, in this case, the kid would likely be screwed, unless he stipulated that he wanted a long-term, high paying job within the company in a higher up position, so that he can oversee manufacturing and sales, and also, innovate upon the original product's design over time. The inventor could also, stipulate that they get a certain percentage of the profit for every item sold.
He could also stipulate that if he goes, then so does his invention, and that they cannot use his idea, or a variation of it, or even sue him for ownership after he leaves the company. They could also stipulate, that they still get a profit from any item the company sold even after their departure.
Keeping in mind the fact that most teens tend to be flaky as well, and also, the company doesn't want to handle the negative PR, after it had been discovered that they ripped off a kid, is why someone who is 18+ would be an even more attractive option. The public would be more sympathetic to a person < 18, even though they really didn't read the contract, and just signed their invention away without thinking about it, than they would to an older person who did the same.
