Infancy (0-3 years old) The period of milk teeth eruption Generally, babies and toddlers start to get their first teeth when they are 6 months old, and 20 milk teeth are fully erupted when they are about 2 years old. During this period, parents should develop the habit of cleaning baby’s teeth with a clean wet gauze or thumb toothbrush after eating and drinking. Preschool period (3-6 years old) The baby’s milk teeth are prone to dental caries because of the love of sweets, at the same time, the baby often has some bad oral habits during this period, which will cause the milk teeth to be uneven, the front teeth to be open (teeth together), and even affect the normal development of jaw bone. Therefore, parents should take the initiative to help their children clean their teeth and do oral examination regularly. School age (6-12 years old) Mixed dentition period, commonly known as tooth replacement period The milk teeth will be replaced by permanent teeth in turn, and this stage often presents an uneven tooth row, which is not easy to clean and easy to suffer from dental caries. Sometimes, the teeth may be misaligned and the bite may not fit properly, so it is necessary to check and treat this situation in time. Adolescence (12-18 years old) The permanent teeth are basically replaced. If there are various kinds of teeth misalignment and bite abnormal phenomenon, you should cooperate with the doctor for orthodontic treatment in time. This period is the best time for orthodontic treatment. Youth period (18-35 years old) The main problems that may occur in this period are periodontal disease, dental caries, bite abnormalities, broken teeth and restoration of missing teeth. It is worth to remind those who have the habit of chewing hard food to pay special attention to the occurrence of oral lesions. Middle-aged (35-65 years old) In addition to periodontal disease and dental caries, a series of oral problems such as tooth wear, gum recession, oral mucosa lesions and tooth loss may occur at this stage, which require us to find out the problems and solve them in time, strengthen the daily oral hygiene care, and choose dental implants to make up for the lost teeth. Old age (65 years old and above) The oral problems in this stage are mainly periodontal disease, dental root surface caries, oral mucosa lesions and tooth loss, etc. Because the elderly are often accompanied by systemic diseases and poor immunity, all treatment in this stage is best to conservative treatment.