A device for determining ageing of a hydraulic fluid in a hydraulic system with a multitude of hydraulic components is provided. The device comprises at least one temperature determination device and at least one ageing determination device, wherein the temperature determination device determines the respective temperature of each discrete fluid volume of the hydraulic fluid in the hydraulic system, and from the aforesaid the ageing determination device determines an increase in ageing. Generally, the temperature determination device carries out a numerical thermal simulation of the hydraulic system, component by component, including determining at least one temperature of at least one hydraulic component of the hydraulic system, which simulation is supported by measuring the temperatures of individual hydraulic components by means of temperature sensors.