There were analysed changes of circadian, circannual and decasecond biorhythms of the test tasks' performance of the control room operators in power stations for the age diapason 22-53 years. The decasecond rhythms' significance of the mental activity rate gradually increases with ageing. The circadian and circannual rhythms' significance for indices of attention and activity rate is minimum in the middle age group and it is maximum in the senior one. The circadian and circannual rhythms' significance for the logic-combinatorial task performance quality is maximum in the middle age group. The revealed differences have been connected with effect of the professional activity specificity on age changes of middle levels of different indices of the operators' mental activity. The complex analysis' results of the biorhythms' significance for the different indices of mental activity are proposed as a (prognostic) criterion of the operators' adjustment to the effective performance of the industrial activity at different times of day and year.