When children enter into Kindergarten at the age of five or six, they will be tested for some basic knowledge. Todays testing standards are much higher than they are in decades past. Preschool can help your child to actually do well in those tests when the time comes.
Children as young as three can begin preschool and do well with it. Although the number of hours and that they have school is very much open to your discretion, almost always it is beneficial to enroll them as soon as they are ready for it. Consider preschool when your child is ready.