I am still nursing my 2.5 year old. He does have caries (at least 4). One tooth is brown. When I discovered the caries, I did night wean. I do not believe that nursing caused this. Although, it is important to evaluate your child's risk factors. My son has a dairy sensitivity, so it is likely that he didn't get enough calcium and vitamin D when he was exclusively breastfed. Now, I think his nutrition is fine because he eats a wide variety of healthy foods and takes supplements. The damage to his enamel had already begun, so now we are just trying to slow the decay enough that his teeth will fall out naturally in 3-4 years. Another risk factor is that my husband and I both have a history of caries.
In sum, every child is unique. I wish that I had taken extra precautions earlier with my son's teeth, but I am glad that I am still nursing him. Other than a few cavities, he is very healthy and happy and smart as a whip.
I will also note that among my many friends who nursed their children, some developed serious caries, and others didn't. I'd keep nursing, but take precautions like impeccable dental hygeine and good nutrition.