My daughter is 11 yrs some of her front teeth r turning yellow mostly by the gumline what could be the problem?

Brushing technique..: The simplest answer is that she is not brushing the gum line area in a consistent manner. It sometimes can be uncomfortable for children to effectively brush this area. I have had success recommended a spinning head tooth brush (does not have to be expensive at all) and encourage her to get those areas.

Staining: I doubt that she would have cavities in these areas at such a young age, but i also do not know what she eats, her oral hygiene habits, or how often you take her to see a dentist. For all the above reasons you should take her for an examination and a cleaning. All the best..

Plaque and Tarter: It is very likely that it is food accumulation as my colleague has described. Children often don't fully understand how to really brush and take care of their teeth. Help them. It would be a good idea to also have the areas examined to be sure it is not something that might need addtional attention as well. Routine professional dental examinations and dental cleanings will also help minimize thing.