Learn Icelandic: Kærastinn hennar er sextán ára. - Her boyfriend is sixteen years old.

No. Unlike the numbers 1 to 4, which change depending on gender and case, the numbers 11 through 20 are completely unchangeable. You will always use sextán for sixteen, no matter what you are counting.

Why is the word for 'years' written as ára?

In Icelandic, age is expressed using the genitive (possessive) case. Ára is the genitive plural of the word ár (year). So, you are literally saying that her boyfriend is 'of sixteen years.'

Why does hennar come after kærastinn instead of before it?

It is the standard rule in Icelandic for possessive words like hennar (her) to come right after the noun. Also, the noun usually takes the definite article when used this way. So kærastinn hennar literally translates as 'the boyfriend her.'

How does the word kærasti (boyfriend) become kærastinn?

Kærasti is a masculine noun. To make it mean 'the boyfriend', you attach the definite article to the end. Because the word already ends in a vowel, you just add -nn to the end, giving you kærastinn.