The first sentence tells us that Gus is not the
youngest. The second sentence tells us that neither
Alessandro nor Flora is the youngest. From the third
sentence, Oliver is not the youngest, whilst the sixth sentence
tells us that Yvette is not the youngest. So Zara must
be the youngest and the order of their ages is Gus, Yvette, Flora,
Alessandro, Oliver, Zara.
This problem is taken from the UKMT Mathematical Challenges.