IMO, the great pyramids of Giza are so old that they predate human history. I think they may be tens of thousands of years older than estimated. The others are simply attempts at replication...thus, you have some that look like kids tried to recreate what was there already..the slope changes, they collapsed, etc..also, when you consider that there are pyramids all along the equator and in other parts of the world, I'd rather say we just don't know, than to try to guess their age, purpose or who built them...I doubt they were used as grain storage...sorry Dr Carson.