The falls at Niagara are about 12,000 years old
The falls were formed when melting glaziers formed massive fresh-water lakes (The Great Lakes) one of which (Lake Erie) ran downhill toward another (Lake Ontario).
The rushing waters carved out a strait (now known as the Niagara River in their decent and at one point passed over a steep cliff like formation (The Niagara Escarpment).From the original falls going over the Niagara Escarpment, the water began to wear its way back up the river.
The path that it left today is known as the Niagara Gorge (a deeply-cut very scenic river path). Eventually this gorge met another cliff forming Niagara Falls.