What Makes A City A Hub?

There any many cities out there that are headquarters to recognizable companies, the home town of popular food chains and the hub of all the action. And, there are some cities out there who aren't known for any of this? So, what makes a city into a hub?
I believe that it is the energy, drive, passion and curiosity of a young professional that makes one city over another a hot spot for hubs and start ups. I think any city who is smart enough to engage their young professionals in community involvement and education is setting themselves up to reap the benefits of that fresh and young entrepreneurial spirit and poof! That city become a hub!
Okay, okay, I know it is not as simple as that. But, how young were Ben and Jerry before they became Ben and Jerry? How old was Nantucket Nectar Tom and his partner before that exploded? Now, these hometown cities can't get enough association with these companies. It can be easily summarized that it is the young professionals that play a large role in making a city a hub and thereby putting that city on the map. How is that for power of the age?