Glacial period (3,3)
'glacial period' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for ice age that I've seen before include "Freezing era" , "'When glaciers covered the earth (3,3)'" , "Period of extreme cold" , "Cold period for Earth" , "Glacial period" .)
(Another definition for iceage that I've seen is " Period of extreme cold".)