I would contest this assertation. Economics as a topic has been present throughout the classical world, although I would argue that the first real "school" of economics was the Salamanca school of Spain in the 1500s. The economist Joseph Schumpeter traced much of early 20th century economic thought to the Salamanca school, including concepts as pricing theory and the quantity theory of money. The reason why economics is commonly associated with the 18th century is primarily due to the influence that Adam Smith's Wealth of Nations had on Anglosphere countries, especially the UK and the US.