In real life situations, and depending on the context, you can use "você" even when addressing people who are older than you. I think that depends on the region as well.
Interestingly enough, in the southernmost part of Brazil, "tu" is way more common as the informal you. And "você" is used in some more formal situations, or when you still do not know that person well (though most of the time "o senhor" or "a senhora" is the proper way to address people who are clearly older). But some people would use "tu" in more situations than other people, depending on how they were brought up and what social class they belong to. Keep in mind that Brazilians tend to dismiss formalities when addressing and relating to other people.
In other regions, "você" is THE informal you and "tu" is not used at all. And the formal you is always "o senhor"/"a senhora".
What I am trying to say is that there are many different situations (in Brazil) and if you try to follow a "static" rule you risk sounding weird.
Oh, I thought when you asked "Por que não" you meant "why not?", as in "why doesn't the boy speak?", which is not really a question about the translation... but moving on...
The meaning is indeed similar, except for the cases when the boy can speak but doesn't want to. The verb "can" is translated to Portuguese as "conseguir" (which also means "to make it", "to achieve"). So the translation for "The boy cannot speak" would be "O menino não consegue falar".
"conseguir" is both colloquial and formal. I remembered now there's also another translation for "can", which is "poder" ("O menino não pode falar"). Also both colloquial and formal. The difference between the two is that "não conseguir" is normally related to a physical obstacle or hindrance (e.g. the boy is mute) while "não poder" usually means that the boy is not allowed to speak. So "(he/she) cannot" is almost always translated to "não consegue" or "não pode".