The idea behind this quote, "democracy presupposes an enlightened citizenry," is not attributed to Thomas Jefferson. In fact, it is attributed to George Washington in his Farewell Address (1796). The entire quote is as follows:
The very idea of the power and the right of the people to establish government presupposes the duty of every individual to obey the established government.
What is being said is two fold. First, democracy requires rational and informed people. Second, these people must abide by the laws of their democracy.
What is provided in the question, "democracy presupposes an enlightened citizenry," is more or less a paraphrase of what George Washington stated in his 1796 address.
Another quote, similar to the one presented in the question reads as follows: "Although a democratic government is founded upon a very simple and natural principle, it always presupposes the existence of a high degree of culture and enlightenment in society." This quote is found in chapter thirteen of Alexis de Tocqueville's Democracy in America.