Learn Croatian: Moj sin je stariji za pet godina.

When comparing things in Croatian, you use za plus the accusative case to express exactly by how much they differ. It functions like the English word 'by' in 'older by five years'.

If za requires the accusative case here, why is the word godina in the genitive plural?

The preposition za applies to the whole phrase pet godina. The number pet itself is in the accusative case, even though its form doesn't change. However, numbers from five onwards always force the noun that follows them into the genitive plural, overriding the case triggered by the preposition.

Could za pet godina also mean 'in five years'? How do I know which meaning is intended?

Yes, za plus a time period often means 'in' for future events. However, because it is paired directly with the comparative adjective stariji (older), the context clearly shifts to measuring the degree of difference.

Can I change the word order to put the measurement first, like Moj sin je za pet godina stariji?

Absolutely. Putting the za phrase before the comparative adjective is grammatically correct and very common. The word order in our example just places a bit more emphasis on the exact number at the end of the sentence.