6-9 months. Once a baby can sit by himself it is easier to hold a cup. This typically occurs when a baby is around 6 to 7 months. Babies are transferring objects from one hand to another at this age as well. Maybe you would like to start with a sippy cup first, but, take the valve out so it does not mimic a bottle. Start wirh small ammounts as for sure some will spill. Be very patient. It is all new to baby.
6 months. Most infants can drink from a "sip" cup by 6 months.
6 months. Most babies can have the cup introduced when they are able to sit reasonably well. Then it usually becomes a matter of practice.
15-18 months. With patience, a parent can train a child to hold and drink little amounts of fluid from a plastic 3-ounce cup. Use little amounts so that when the child spills, there is little to wipe up.