When did Ice Ages Occur?
Several glacial advances and retreats have taken place in the last billion years of Earth's history. These glaciations are centralized in four different time intervals. Very large and lengthy glaciations occurred during the late Proterozoic (between about 800 and 600 million years ago), during the Pennsylvanian and Permian (between about 350 and 250 million years ago), and during the late Neogene to Quaternary. Less extensive glaciations occurred in the Ordovician and Silurian (between about 460 and 430 million years ago).
During each of these glaciations, many glacial advances and retreats were occurring. Over 20 glacial advances and retreats have occurred during the last 2 million years.
The term "ice age" means "a long, generally cool, interval during which glaciers advance and retreat". This description sounds similar to our climate today, and therefore we are still currently in an ice age today. Our modern climate represents a very short, warm period between glacial advances.