As a single data point: I have been able to reliably score an 800 on the SAT Math section since I was 14 or so (I'm 16 right now). Seeing as the SAT has shifted from the 1600-scale to the 2400-scale, it's possible there are some differences, but looking at the problem example provided in the article, they feel more or less identical to me, so I don't think it's beyond making comparisons. I don't find SAT math very difficult at all, and I don't make careless mistakes on the problems either, which does suggest to me that it's easier to avoid careless mistakes on problems you find easy. On the other hand, I find the last 5 or so questions on the AIME (American Invitational Mathematical Examination) absolutely hellish in terms of difficulty, and I've noticed a significant proportion of careless errors in my work when doing those problems.

I'm not exactly sure why this is; intuitively I'd expect there to be little to no correlation between the difficulty of the problem itself and the difficulty of avoiding careless mistakes when doing those problems, but clearly this is not so (at least in my admittedly extremely limited experience). The best explanation I have right now (which is almost certainly a just-so story, but whatever) is that humans have a limited amount of "concentration ability", and devoting more concentration to doing the problem results in less focus on avoiding careless errors, or vice versa.