Somewhere between 5 and 9 months would be the best time to introduce your child to its first cup. Because every child is unique, the talents and interests are different.
Are you noticing that you child isn't interested in the cup?
Don't push it and try again later. Learning how to drink independently is an new, important step in growing up. It takes a lot of exercise and making a mess is just part of the game. Make sure that you child is always sitting in an upright position when drinking. This prevents choking.
The drinks you put in the cup are equally important. Water, milk and fruit juice without added sugar are recommended.