In general, when you're a teenager, your ability to construct effectively timed and worded persuasive arguments is limited. You wouldn't be able, nor be expected to, do things like argue for a pay-rise, pitch your startup to an investor, or keep up a professional correspondence in project management.
This is a stage to learn the ropes, and be aggressive in expressing one's opinion. In the lingo of Jeff Bezos, now is the time (generally) to be clever, offending some people if you must. Those who stay silent never learn the ropes, and lead a life dictated by other forces other than their own.
As an adult, especially in a professional or sensitive family situation, knowing what not to say is arguably much, much more important. Worse still, most of the time you don't even know how much harm your inaptly timed words have done, making it hard to learn from past mistakes. Iterating on past experience ceases to become a viable strategy.
Watch how the words of the many screw them up. Watch how the silence of the few drive them forward. And shut up until you think you've got it right.