Why did the Ice Age start

South America finally met up with North America with the completion of formation of the Central American isthmus, which was about 5 million years ago (process started roughly 7 million years ago, and finished after a while). This resulted in the cutting off of major equatorial oceanic current between the Atlantic and Pacific Oceans, resulting in a drastically cooling effect on the planet as a whole.

Answer:

There have been five major ice ages in the Earth’s past.
The Huronian, 2.4 to 2.1 Ga (billion) years ago during the early Proterozoic Era.

The Cryogenian period) 850 to 630 million years ago.
The Andean-Saharan, A minor ice age occurred from 460 to 430 million years ago, during the Late Ordovician and the Silurian period.
The Karoo Ice Age occurred from 360 to 260 million years ago. Potentially caused by a depletion of CO2 by generally lush vegetation.
The Pliocene-Quaternary glaciation (the current ice age) started about 2.58 million years ago during the late Pliocene.

Aside from speculation on potential causes no definite comclusions have been reached.