The Vibe in New York City

I didn't write anything since Thursday because I've been busy traveling. 

I'm currently in one of the cities that claims to be "The City That Never Sleeps". Yup, I'm currently in New York City. 

Every time I come here, a visual aspect of the city changes a bit. Stores come and go. But, the pulse of the city remains the same. I wonder why is that? I've read reports about how New York City is mainly a city of young people as older people look elsewhere to retire or live in peace (and not surrounded by this frenetic energy). Is that the secret of New York City's pulse - an abundance of young people? But Tokyo also has this pulsating vibe and that's a city with a graying population. I don't think it's population density since highly populated places don't necessarily have this vibe. What's the "x" factor for cities like NYC, Tokyo, and Hong Kong?