I reckon its the late ‘80s and early ‘90s. Discuss below.
Vintage muscle: Raw power, carburetors, bias ply tires, no driver aids or safety whatsoever, wet noodle, rode like a truck compared to today.
Modern muscle: Controlled power, modern fuel injection, large concessions to safety, handling, driver aids, and interior quality.
Mid-age muscle: Somewhere in the middle on all aspects (except power - which is lower than both - but so is the price.)