Actually your question is not clear. There are PAT exams held in different states and for different courses. There are slight changes in the eligibility criteria everywhere. It would be better if you restructure your question and ask agin including the state and the course you prefer. In general, the candidate must be with Indian Nationality and there's no upper age limit for the exam. The candidate must have passed class 12th with Physics, chemistry, biology/ mathematics/ biotechnology accordingly. So, you can apply after you pass class 12th. Hope this helps.