You can start preparing for UPSE exam from your 6th standard if at that age you are mature enough to think of it. Otherwise, the best age to start preparing is when you are in 11th standard, choose Humanities stream and study hard. Pursue your graduation in BA Honours with any of the four subjects either Political Science, Geography, History or Economics and prepare for the exam with full dedication.
This is my personal opinion for preparation of UPSE Exam.