Why is your teeth yellow?
Your teeth being yellow is due to the thickness or thinness of your tooth enamel showing the cementum under. Don't expect it to be really white for those who bleach teeth, its perfectly normal to have somewhat yellow teeth.
As a general rule, teeth start out white and become more yellow with age, therefore it would be unusual although not impossible for kids to have yellow teeth. Kids can have yellow teeth if they indulge in habits and diets that cause yellowing (and even browning) of teeth - such as tobacco, coffee, not brushing, bad diet, etc. Malnutrition can also bring about yellow teeth in children.
No, not brushing your teeth will make them yellow but it will make them white and if your teeth where already white it may make the blocks yellow when you take them off also if you eat sweets and chocolate anything like that or drink sugary foods can make your teeth yellow but be careful with your teeth cause it may happen .
Some people's teeth turn yellow due to smoking, I don't smoke and personally don't know a lot on the subject but smoking for a long period of time or having high amounts of cigarettes a day can lead to yellow teeth. Yellow teeth is also caused by decay and not brushing your teeth. Eating too many sweets/sugary foods can also lead to yellow teeth and especially if you don't brush your teeth.