Complex sentence: The European Union Science Olympiad (EUSO) is a competitive team contest for secondary school students from the European Union Countries, 16 years of age or younger on 31 December in the year before the competition.
Split into simpler sentences: The European Union Science Olympiad (EUSO) is a team contest for secondary schools from European Union countries. To participate in EUSO, students must be 16 years of age or younger on 31 December in the year before the competition.