Careful! This means the speaker is *exactly* 18 years old, surely this was not meant by the English sentence“I am eighteen years old.”.
But usually, when someone says to be X years old in English, rarely the exact age is meant nor is it rounded to the nearest whole number. Actually what is meant is how much time one's birthday has happened so far.
So “I am 18 years old.” actually means “It now happened 18 times that the Earth revolved around the Sun after I have been born.”.
pei nauku