Leaving aside caste systems, as a bit obvious, I will point out that at law, it may be illegal for the character of noble birth to engage in certain occupations, or it may bring down any legal privileges he has, so it's a major trade-off. Taxes, for instance.
But past that, there was the little matter of getting an apprenticeship without the aid of your family -- which, to be sure, would have a much wider range than nowadays. Quite distant cousins could be pressed into assistance. Or inheriting the family business, which would be obvious. There were a certain number of jobs as we conceive of them now, but in medieval times and even later, the expectation was that adults had married, lived in their own household, and were self-employed, using or selling the fruits of their labor. (Nineteenth century discussions of "wage slavery" were often fraught with worry that wage-laborers could not be properly independent adults.) Being paid for working was something that youngsters did -- in theory. In practice, of course, there were adult wage-laborers in medieval times.
Setting out to seek your fortune had to be done sometimes, of course, but finding a place was difficult even in fairy tales. And there is of course the tart little moral that Perrault appended to Cinderella -- all sorts of virtues are like to prove useless, if you don't have that handy connection, a good godparent.