As same as in Meteorology, in Astrology also, there is no definite saying. Because many things can not be told hundred present accurately. Life is a very complex thing many influencing factors are taking part during the course of a persons life It is dynamic, constantly changing just like clouds and weather patterns.
Only a divine entity or God can predict for 100% accuracy about the course of a life of a person. No being born as a human can predict with 100% surety about course of a life of a person.
Because a persons life is governed by Karma and so complex the karmas are it is difficult to fully comprehend by a person with ordinary intellect.
Thanks to the works of Irshis and Sages and all the other scholars of the of the past, about 75 - 80 % degree of accuracy of predictions can be achieved by a good astrologer at present, if more than that, the rest is pure chance.
No good astrologer who is honest and who knows the subject of astrology well and has a reputation to protect will never mislead people who do not know astrology and dare to say that he or she can give perfect 100% accurate predictions.
It is as same as that, even for a vehicle which is just out of the production line, a good master engineer will never give a 100% guarantee, say to cross a desert alone and without taking any spare parts. Because he knows very well what many things can go wrong and how many brand new vehicles had been recalled due to various defects. But at the same time a foolish driver who may even do not know how to change a plug, due to his foolishness may take the risk of crossing the desert or even declare the exact time he will arrive at the destination.
Since the Astrology is the only tool an ordinary human being has, to interpret the Quality and the quantity of Karmas an individual posses, Astrology should be treated with respect. Even though it's accuracy is not always 100%.