True sharing involves empathy, the ability to enter the other's mind and see things from their point of view. Children are rarely able to have true empathy before the age of six. Before that time they share because parents condition them to do it but not because they really want to do it. Don't expect a child under the age of two or two and a half to readily agree to share.
Children under the age of two are in parallel play: they play together with other children, but not with them. They worry about themselves and their possessions and do not think about what the other child wants or feels. But, given guidance and generosity, the selfish two-year-old can turn into a generous one at three or four years. As children begin to play with each other and cooperate in play, they begin to see the value of sharing.
Children with parental attachment may be more sensitive to the needs of others and therefore may be more willing to share. Or they may also be more aware of their own need to preserve their sense of self by not sharing. It is easier to share with someone less powerful than you or less threatening (i.e. a younger child), a visitor rather than a sibling, a quiet child rather than a demanding one… Much depends on the child's temperament.
To know if your child is ready to share, you should see if he is really ready without judging him. Even when a child is three or four years old, they expect a fair exchange. Respect and protect the right for your child to have his own possessions. Your child may not want to share his toy because he is fond of it and knows that the other child may not take good care of it and may even break it. A child can begin to share from 3-4 years, but it will not be after 6 when they do it with pleasure.