1. A method of operating a wind farm including plurality of wind turbines with a plurality of turbine components comprising a first and further turbine components, said plurality of turbine components subject to degradation, comprising: predicting, for a turbine component of a first wind turbine and for each of a sequence of time intervals (t 1. .. t N1 ) into the future, based on a sequence of first turbine control input values (u 1 (t 1 ). .. u 1 (t N1 )) including a component maintenance action at a maintenance interval tM 1 , a component life index L(t) of the first turbine component, determining a sequence of optimum turbine control input values (u 1 *(t 1 ). .. u 1 *(t N1 )) including a) an optimum maintenance interval t Ml * for the first turbine component and b) optimum maintenance intervals t Mj * for each of the further turbine components that optimize an objective function J(u) depending on the component life index L of the first turbine component and on component life indices L j of the further turbine components, and operating the first wind turbine according to at least one optimum turbine control input value u 1 *(t 1 ), wherein the method further comprises predicting the component life index of the component based on a sequence of second turbine control input values (u 2 (t 1 ). .. u 2 (t N2 )) of a second wind turbine electrically connected to a same branch of a collector grid of the wind farm as the first wind turbine, by evaluating voltage phase and amplitude difference between the first turbine and the second turbine, and determining optimum first and second turbine control input values u 1 *(t), u 2 *(t).