User blog comment:Kevo411/HxH Questions 2/@comment-14902757-20140615074850

I guess Jaadowgg & FinalFlash18 pretty much answered your questions.

The hardest question is #4 and I think Jaadowgg made the right point about Kurapika and the Troupe. There's quite a similar discussion about this a few months ago but here are some reasons I could think of right now:

1) You'd imagine people who know how to use nen to keep their aces up their sleeves as a secret.

2) People may know about it but not all can even begin to understand how it works and how it's used. Put it this way: you know some people can paint realistic images but you can't no matter how hard you try so you end up just minding your own thing.

3) Only conjurer abilities are visible to the naked eye. And they won't look so unusual unless if all abilities are like Kite's Crazy Slots, which is a high level conjurer ability you won't see being used recklessly in public.

4) In episode 83, it's shown that the Hunter Association (via Chairman Netero) follow orders from the world's leaders. I'd imagine that in the Hunter x Hunter universe, there are nen users who work to keep the peace and order as well, that's why there are also types of hunters dedicated to hunting criminals. 

About Killua not learning nen at a younger age, FinalFlash18 shared the link to a past discussion in his answer to #3. Just pick which of the answers convinces you the most.