Even a seemingly simple thing such as drinking from a cup needs practice: it may well take two to three months. Therefore, it is reasonable to start with it during the seventh to eighth month.
You can make drinking out of a cup easier for your baby: it is best to use a plastic cup at first, as these are unbreakable. Hold the cup yourself in the beginning and only offer your child small amounts of liquid at first. When your child can hold a cup him/herself, it is recommended to use a special drinking cup for learning to drink and to let the child drink on his/her own.