This is the 2899th most frequent Dutch word.
achttien
The number eighteen.
Here, "achttien" functions as a cardinal number referring to the age of the speaker.
Ik ben achttien jaar oud.
I am eighteen years old.
In this sentence, "achttien" quantifies the number of apples, used as a cardinal number.
Er zijn achttien appels in de mand.
There are eighteen apples in the basket.
The word "achttiende" indicates ordinal use, pertaining to the sequence of Sara's birthdays.