What Am I Riddles

Question: Looking at my face, I'm no more than six.
Look at the rest of me and I'm 21.
You'll see my twin and me in Las Vegas.
Say my name and you'd be uttering a threat.

What am I?

Question: Occurring once in a hundred years and once in thousands. But never in a day or a year or a century.

Question: Having one, means having something to follow. Once you question it, it means having none. People seek it, but some do and some don't find it. What is it?

Question: I can be flipped and broken but I never move. I can be closed, and opened, and sometimes removed. I am sealed by hands. What am I?

Question: People pick me, and watch me until they give me power. Then they don't stare in my eyes. What am I?