According to US Baby experts:
You can have your baby use a teether in their teething period from 18 months to 2 years old. By so doing, it can relieve discomforts such as gum acid and itch while massaging your baby’s teeth.
Primary teeth will erupt when your baby is around 6 months old. From that time, you can help your baby clean teeth. Fix a primary-tooth brush on your forefingers to clean your baby’s teeth. The toothbrush should be made of food-grade liquid active silicone, soft and tender so that it won’t hurt your baby’s teeth. In addition, a funny design will make your baby enjoy teeth brushing unconsciously.