I am undecided between an economics major and an economics joint major. When do I need to make this decision?

Because the study of economics is highly structured at the beginning, all of the majors, concentrations and joint majors start with the same basic courses. The differences between the majors arise when you begin to take electives and seminars in the department. So you can begin your study of economics and choose your particular major only as your interests develop. Once you have selected a joint major, make sure that you are familiar with the requirements as they pertain to your particular major. Note that some courses are accepted towards the economics major that are not accepted for particular joint majors. In particular, the joint major with philosophy and the financial economics major have restricted lists of electives and you must select your electives from that list so you should read your requirements carefully.