How often are brain cells replaced?

Every cell in the human body is replaced and renewed within a period of seven years, consecutively, for life. This is known as aging; it includes the brain. Not one cell a person is born with is still there when they reach seven, and again at fourteen, then again at twenty-one, etc. The cells are replaced, respectively, and you are a "brand new" person, however, with the same DNA structure and personality you were born with.

Memory cells can be "recycled" as some information is lost over time.

Sleep repairs and reorganizes the brain; as for new brain cell development, research shows that as one educates their mind new cells form as often as the mind is actively engaged.

EDIT - MijinLaw: The 7-year cycle is a myth. And although neurons can be generated in some circumstances, it's still unclear whether neurogenesis plays any significant role in normal learning.

Plus! Heart muscle cells aren't replaced, that's why heart attacks can be fatal, as the muscle cells which are destroyed are not replaced and may not be enough to keep the heart going. There arnt!!!!!!!!!!!!!!!