Recent optical thermolumensce (when 2 pieces of stone were last joined) tests seem to show that they may be far older than standard theory posits (by hundreds of years). All dating of the Old Kingdom is based on two Kings Lists - the most commonly used is a known forgery, and the other seems to be something of a copy of the first.
But one thing is sure - none - that is none of the pyramids were used to store anything, let alone grain. The Egyptians had huge storehouses for that purpose; going to the trouble and expense of building a pyramid - of all things - to store grain is ridiculous. For the same time, trouble, and expense of building a structure with extremely limited internal space when you could otherwise build many storehouses is just plain out there with the aliens.
The Step Pyramid of Djoser could be younger that the Giza pyramids, not the other way around. The Giza pyramids were likely build by the Westerners who ruled that area until the war-liking southerners conquered them a thousand of year later.