Disagree strongly. Never too early to start UMAT prep - in fact, you have probably already starting inadvertently preparing to some degree. Many people are very short-sighted about what UMAT preparation entails. You don't have to start any questions or subscribe to any courses in year 10 but you can do many things to start preparing. Read the paper on a daily basis, take up chess on a competitive basis, start taking up sudoku - give debating a shot at school, start reading some novels - more than just the prescribed ones from your school. Just some suggestions.
Also I have to particularly disagree with Xenial. While I can commend his/her natural skill, it is wrong to assume everyone else will be able to handle the UMAT in the same manner. Personally, as a UMAT tutor for 4 years, I have seen countless people drastically improve through study and focus - some quickly, others through months of work. One thing is given, and I have noticed this in all cases, when guided and where you can obtain some form of mentor, even if a friend or former student, persistent UMAT training will result in improvement.