Soviet story goes that the competition leading to SU-25 was launched 1969, Sukhoi being appointed the task in late 1969. It had been on the drawing board already on 1968 but the comperition made it official. They made many versions of the T-8 throughout the 70"s eventually leading from T-8-1 to T-8-D in 76-77 with new wings to relief loads on stick with lots of other modifications.
Eventually first AC saw action in Afghanistan in summer 1980.
To make it short, the idea was there already in 1968 within Sukhoi office. Officially it was started in 1969 (from government perspective). However it took more than ten years to have one flown in combat with lots of midifications from the original.