Hi. Everybody's teeth become more yellowish with age. Its related to our lifestyle, diet and normal ageing process. If you want to prevent it, avoid smoking, drinking tea and coffee excessively or consuming coloured foods. Since this is not possible entirely, it is a good practice to rinse thoroughly after any of this. There are many whitening toothpastes available over the counter, but they have limited effectiveness. You may opt for tooth whitening treatments if it is a major concern.