From Old French -age, from Latin -aticum.
Cognates include Spanish -aje and Italian -aggio.
word-forming element in nouns of act, process, function, condition, from Old French and French -age, from L.L. -aticum "belonging to, related to," originally neuter adjectival suffix, from L. -atus, pp. suffix of verbs of the first conjugation.
1.The sound change from "t" to "g" is too obscure to me, and how to explain this phenomenon reasonablly?
2.Is there any other word undergone the same change?