周岁 (zhōu suì): one full year (e.g. on child's first birthday)

Today the baby turns one year old. We bought him a big cake and took many photos.

In China, when a child turns one year old, many families hold a 'zhuazhou' ceremony to predict the child's future career.

My daughter is already two years old, but she still can't speak much; she can only say 'papa' and 'mama'.

According to hospital regulations, after a baby reaches one year old, several vaccinations are required.

As soon as Xiaoming turned three years old, his parents sent him to kindergarten, hoping he would learn to get along with other children early.