The syllabus of RMO and INMO are quite different from the syllabus of IIT-JEE with only a few topics overlapping. Hence, a student of Std. 11th / 12th does not have a distinct edge over a student of Std. 9th / 10th. However, in case of other Olympiads, the syllabus is quite similar to that of JEE and hence a student aspiring to clear the 1st level must complete the entire syllabus of JEE Physics or Chemistry to stand a chance. Also, Std. 12th students are debarred from appearing for Maths Olympiad( but are allowed to appear for Chemistry and Physics olympiads ), making it even easier for students of Std. 9th and 10th to have a good chance at clearing RMO.
The above reasons ensure that each year, quite a few students from Std. 9th and 10th clear Maths Olympiad while students mostly clear Physics and Chemistry olympiads primarily in Std. 12th and occasionally in Std. 11th. In fact, last year, a 9th Std. student went on to represent India in IMO( International Maths Olympiad ).
