From what I have read this may have something to do with social mobility due to the industrial revolution. Originally, the reason why people typically took up their parents trade was because it was available. It wasn't often people would jump from the poor lower class into the upper class because there was no social mobility. During the industrial revolution the middle class appeared because people were getting jobs doing things never done before. The class system worked, before the industrial revolution, in a way that there was just a poor lower class and a upper class, with only a few exceptions of people who fell in between. After the industrial revolution because job opportunity in cities sky rocketed with new inventions and factories being built, people began to move to cities to pursue higher paying jobs. Although those jobs in the long run probably were very taxing and somewhat dangerous, many people were able to move up in class and leave behind the old family trades. Hopefully this answers at least part of your question as to "when" it all began.