[2] According to oral tradition, the Aja migrated to southern Benin in the 12th or 13th century from Tado on the Mono River, and c. 1600, three brothers, Kokpon, Do-Aklin, and Te-Agbanlin, split the ruling of the region then occupied by the Aja amongst themselves: Kokpon took the capital city of Great Ardra, reigning over the Allada kingdom; Do-Aklin founded Abomey, which would become capital of the Kingdom of Dahomey; and Te-Agbanlin founded Little Ardra, also known as Ajatche, later called Porto Novo (literally, "New Port") by Portuguese traders and the current capital city of Benin.

[3] Currently, there are approximately 500,000 Ajas in an area straddling the border between Benin and Togo, 50 kilometres (30 miles) long and 30 km (20 mi) wide.

Scarcity of natural resources, overpopulation, and chieftaincy disputes contributed to the separation and dispersal of both the Ewe and later the Fon from Adja.

Many are trilingual, also speaking French and Fongbe, the lingua-franca of southern Benin, while Ewe is spoken as a second language by those Aja living in Togo and Ghana.

There are a significant number of Aja living throughout the coastal region of Benin and Togo, southern Nigeria and Gabon.