In general, but being completely dependent on the personality of the older child, children 4-6 get frustrated playing with a child that much younger. After about 6-7, the older child would most likely see the younger child as being younger enough that he is not a playmate, but more of someone to take care of and entertain, like a mama does. Which can work just fine as long as the adult is supervising pretty well.
The difference in skills in playing are so much more profound when they are that young. The 4-6 year olds are moving out of the chaos stage - where blocks were basically great for building towers and knocking down, for example, to now being useful for building roads and structures for cars and dolls, with rules and scripts and all that. Its hard to be at the age where you are just beginning to enjoy that kind of play but your playmate can't grasp those concepts yet.
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