Learn Croatian: Ona ima četrnaest godina. - She is fourteen years old.

In Croatian, you do not say that you are a certain number of years old. Instead, you say that you have a certain number of years. Ona ima literally means 'she has'.

Since she has multiple years, why doesn't the word for year end in '-e' (godine)?

This is where the rule for numbers 5 and above comes in! While you would say dvije godine (two years) with numbers 2, 3, and 4, any number from 5 onwards requires the following noun to be in the genitive plural.

But isn't godina the singular form for 'year'? Why use it with a number like fourteen?

It looks identical, but it is actually the genitive plural! The nominative singular (one year) is godina. By coincidence, the genitive plural form used after numbers 5 and above is also godina. You can always tell which one it is from the context of the number.

Does the ending -naest in četrnaest work like 'teen' in English?

Exactly! Just like English adds '-teen' to numbers (fourteen, fifteen), Croatian adds -naest to the base number. So a slightly shortened četiri (four) becomes četrnaest (fourteen).