An active matrix display device has an array of oled display pixels (2) operable in two modes in which the power supply line (26) is modulated between a low voltage and a normal power supply voltage. In a first mode, a pixel drive transistor current is supplied to the display element (2) and is selected to provide a desired pixel brightness. In a second mode, a voltage is provided to the drive transistor and is selected to provide a desired ageing effect, but no current flows through the display element. The frame time is thus divided into two periods, one when the power supply line (26) is supplied with a voltage of e.g. OV or -5V to turn the display element on and the other when the power supply line (26) is supplied with a voltage of e.g. OV or -5V to turn the display element off. During the off period, a voltage is nevertheless applied to the drive transistor, and this voltage is selected so that the overall threshold voltage drift in the drive transistor for all pixels (resulting from ageing) is substantially the same.