"Вам вже вісімнадцять років."
Translation:You are already eighteen years old.
I guess the person stating the comment doesn't 'know' the young 18 year old person well because they used the word 'вам' instead of 'тобі'. (Unless there was more than one 18 year old person presesnt.) An older person would address a younger person as 'Ти'/'тобі' not 'Ви'/'вам'. Just an observation.
In business communication people usually adress to each others with "Ви","Вам" despite the age of speakers f.e. teachers at my university never asked me with "ти", "тобі" even they were professors