If you are strong minded and sure about what you want in life, then you should declare asap. If you want to go to college and kind of test out the water to see what you excel in, you may declare in junior year. But i would suggest to do it sooner than later because in some schools, it's really difficult to transfer into another major, it will require like a 3.8 GPA and you have to go through all these processes to get accepted.