According to the article they have been isolated for thousands of years.
Perhaps they don't have the same DNA for language as other humans do. Wikipedia said they don't have the ability to count, even when taught.
http://fora.tv/2009/03/20/Daniel_Everett_Endangered_Language...
In this interview, Everett mentions that he met a 13 year old girl in Brazil who happened in fact to be a Pirahãn—working as a cashier clerk, fully well able to count and do maths.
I can only guess, but I wonder if yours was downvoted for making claims that seem wrong without backing them up - I don't think there is an age when new connections are no longer made nor do I agree that adults can only learn from what they already know - but I wouldn't vote it down because of my disagreement.
Though there has been recent discussion on people voting down purely for disagreement, and that might well violate etiquette. But just <0 without a comment doesn't necessarily do so.
This is why e.g. bilingual kids learn new languages much faster than people who only speak one language, people who learn an instrument as a child can pick up new ones much faster, etc.
What is a personal theory of mine is that if you don't have language to understand a concept by a certain point you can never truly understand it. The only places I point to that support my theory are the few cases of children being raised away from humans [1] and this Pirahã story. It's pretty difficult to prove scientifically since it would require destroying someone's life.
But on this site, of all sites, I would expect discussion about it, not a simple down vote.
[1] I've heard of at least 2 seperate cases where through some odd circumstanced a child reached teenage years without learning any kind of communication with humans. After they were rescued, in both cases they were never able to grasp concepts much further than a young child (around where chimps who learn sign language get to). This is by no means conclusive, since they obviously went through an incredibly traumatic experience, but as I mentioned above, there is no easy way to apply scientific method to this theory.