I am Ann, mum to a twelve year old son.
Recently I have noticed that a few of the front teethes of my son have become yellowish. He himself brushes his teethes from a very young age of around 3 (first few months I helped him). He brushes twice a day in the morning and night. He is not having any pain or complaints.
What are the reasons for this? Is it due to excessive intake of Calcium or any other food? Do I need to consult a dentist?
There are a number of causes of yellowing teeth. If a child drinks fluoridated water during his tooth forming years, the flouride damages the enamel-forming cells resulting in teeth that have a porous surface and the teeth turn a dingy yellow.
Tetracyline is an antibiotic which is contained in cold medication and can cause yellow teeth if ingested during the time when your child's teeth are forming.
Some foods and drinks are highly pigmented and will result in yellow tooth staining. Blueberries, cherries, soda, soy sauce, and coffee are all examples of highly pigmented foods. If your son regularly eats these foods (soy sauce?) the teeth can turn yellow.
And finally, not brushing and flossing regularly in order to remove plaque and staining substances can cause teeth to yellow. This is the one I think is probably causing the yellowing in your son's case, being that he is 12 years old and likely isn't diligently brushing and flossing twice a day.
It is important for your son to rinse his mouth with water after eating highly acidic beverages (sodas) and highly pigmented foods, and he should brush his teeth well (set a timer for 3 minutes) right after breakfast and right after dinner.
If your son is seeing the dentist every 6 months for a cleaning, then you probably don't have to make a special appointment to see the dentist for yellowing. If it's been more than 6 months since he had a cleaning, then I would take him in for a professional cleaning that may actually help to remove some of the plaque that could be causing the yellowing.
Here's an excellent page that discusses the causes of yellow teeth in more detail.