|Little Hippo (LH)||Students born in 2011 and younger. (2011<)|
|Hippo 1||Students born in 2009 and younger. (2009<)|
|Hippo 2||Students born in 2008 and younger. (2008<)|
|Hippo 3||Students born in 2006 and younger. (2006<)|
|Hippo 4||Students born in 2004 and younger. (2004<)|
|Hippo 5||Students born in 2002 and younger. (2002<)|
|Hippo S1 (Special Category)
-for students in Private English Schools
|Students born in 2007 and younger. (2007<)|
|Hippo S2 (Special Category)
– for students in Private English Schools
|Students born in 2002 and younger. (2002<)|
The students of lower grades than the ones in the table can also compete if they feel prepared for the level. The age limit for the Olympiad is 19 or as long as the students are in high school.
Little Hippo category students compete in the regular category independent of the type of school they come from.
Special categories are aimed at students who are considered bilingual according to the Hippo Olympiad regulations as well as for the international schools and the schools where the majority or all subjects are taught in English (except for the countries where English is the only official language, e.g. UK).