Every student has some goal like becoming an Engineer, Doctor, CA, IAS, Designer, Lawer, etc. The list can be quite big. For any of these, one has to take some competitive examination either for admission in some course or to qualify for job or for both. At some point in school education, one starts planning to take the required examination at later stage to pursue his goal. If someone wants to become an Engineer, he starts thinking about IIT or engineering admission tests. Similar options are available for other goals as well. Lots of effort go into finding answers to questions like where to prepare, when to prepare, how to prepare etc. Also, the focus remains on getting good marks in the school examinations while rest of the things related to learning wait for the time to come. For example, if a student studying in Grade 8 is learning "Ratio", he will learn definitions of related terms, formulas etc., solve many questions depending on the examination pattern and may try to do these fast. The same student may have a goal to do Management Studies from the most prestigious Institute in the country for which he will require to compete in an extremely competitive examination in which quantitative aptitude has good weight. That may require good understanding of concepts and application related to "Ratio" in various areas involving few more topics that he is studying in the same Grade. But that examination will be after few years. Most of the students focus on short term goal and study for school examinations. Later, leave themselves with no other alternative than to do most of the topics afresh while taking the competitive examination. Studying "Ratio" for school examination and for competitive examination should not be different as the concepts remain same. Change in pattern of examinations or questions can be handled easily if the topics are studied in right way with proper understanding of concepts and applications. Topics that we learn at school level are required at later stage as well. So, why should this question be there about when to start preparation? Why not study the topics properly keeping long term objective in mind!
You can see various elements in the above picture. They are in fall colors. All these leaves are so colorful. Did they get all the colors in fall season? When did they start becoming so colorful? They have gone through various stages of development and have done well in all the stages. Those who didn't do well in earlier stages had fallen much earlier. Strategy of doing something right when the time will come doesn't work. Doing right is a habit. If you procrastinate, work to somehow manage short term goals most of the time and study the topics just for an immediate examination then it will be very difficult at later stage as the study load will get accumulated beyond manageable level.