Pick something to get good at, then really work to get really good at it. The younger the better. But be focused. Ideally something you can make money with.
IMHO that's a surefire way to burnout and self-doubts later on. My advice would almost be the opposite.
Never too late to change if what you're doing isn't working for you. Recognize when you're about to kill your passion with expectations, and don't do it. There is little to no cross-disciplinary knowledge that doesn't come in useful, so don't force yourself to be single-minded in your pursuits. What you're learning matters surprisingly little, that you're learning matters so much more.
But yea, don't change major pursuits, like, every year. Probably depends on the person which advice they need. I definitely would have needed the latter.