I'd say it crops up earlier than 5 years old. I've got a 13-month old son and he constantly multitasks. We can't stop him. He walks around banging on things while he's drinking milk from a cup. He fiddles with a toy while reading a book.

He's definitely capable of very focused attention, but it seems like once he's mastered some simple thing he gets restless about doing just that and nothing else ... he'll start wanting to combine it with something else. Sound familiar? :-)