"He is a child, you are a man."
Translation:O bir çocuk, sen bir erkeksin.
That subject is called a "personal copula" or just a personal ending. If the subject is in the 2nd person. you add the subject -sIn, and if it is in the 3rd person, there is no suffix.
The subjects in the two parts of this sentence are different, ergo they have different suffixes. :)
I see people saying that the third person doesn't need a suffix, but is it incorrect to use one? Duolingo gives us a third person "to be" suffix here: https://www.duolingo.com/skill/tr/To-be/tips-and-notes