What does Nick mean when he describes Jay Gatsby as being the type of man "a seventeen-year-old boy would invent"?

The extravagant parties, luxurious car, the vast amount of wealth. Everything that is Gatsby is extravagant and almost seems to be imagined by a seventeen-year-old's stereotype of a multi-millionaire. That is why Nick describes Gatsby as being the type of man "a seventeen-year-old boy would invent".