In 2000 years, someone will write this exact same essay about American football.
http://www.basketball-reference.com/contracts/players.html
http://blog.futureadvisor.com/nfl-players-must-go-long-on-re...
Not actually true, at least directly. They're currently required to play at least one year in NCAA, and the NBA commissioner is advocating for two years.
International players can play professionally as early as 16 and then immigrate over. A few top US prospects have also gone overseas @ 18 for 6mo/1yr to bypass the NCAA restriction and start playing as early as 19.