Shimano's 1982 catalog had "Aero" levers for the AX groups, standard levers for the EX groups. There was no "exact" year when the transition happened since all manufacturers were fishing around the "aero" movement in the early '80's. However, by the mid-'80's mountain bikes had taken over the market and road bikes became almost irrelevant.

Shimano 1982 Dealer Catalogue