The sentence in question is "Ей ещё нет и тридцати." and the translation given is "She is not yet thirty." (well that in German, because that is my native language). I do not understand what exactly the "и" is doing in this sentence? Why is it there?
The grammar section just explains the basic examples, i.e. that you put the person in dative + number + год/года/лет, depending on what the number is. And I understand that number in the example above is genitive above because of the negation, "ещё" is to express the "yet" and that "лет" can be omitted just like in English if the context is clear, but not the "и".