Is it ever too late to improve? Of course not. At least I don't think so. As soon as a student decides THEY want to make more of an effort things can start to turn around for them. I just bumped into a former student today that is just about to graduate college. This particular student really decided to take it up a few notches once they got to college. They started really digging into their math now that they had a clearer picture of where they were going and that math was a hurdle that they needed to get over to get there.
There are so many factors as to why a student doesn't excel in math. It could have to do with a student's early experiences with math that caused them to perceive themselves as 'bad' at math. It could be a lack of effort. A bad repoire with their math teacher could be a deciding factor. A lack of understanding the fundamentals or a poor background with previous math courses can be one's downfall, as well as many many more possibilities exist.
Almost regardless of the cause, a student can begin the process of turning things around. Numbers are not going anywhere anytime soon. It's not like you are trying to keep up with a moving target that keeps changing every month or year. So, realizing that, you can begin to go back over what you don't understand with patience and calmness and with a new perspective. What you didn't get before now you can begin to understand which will in turn make other concepts that depend on those fundamentals easier as well. You will begin to fill in the gaps and be able to see how the pieces of the whole fit together. Start where you are and improve from there. The best time to plant an apple tree is 20 years ago but the second best time is today, as the saying goes!
Helping students succeed in math for over 15 years. Individualized attention makes the difference!