How to say how old you are.
Would the phrase "17最です" work to say "I am seventeen years-old"?
Or does it have to start with 私は?

Question

How to say how old you are.
Would the phrase "17最です" work to say "I am seventeen years-old"? 
Or does it have to start with 私は?

Answer

It does not have to start with 私は. it sounds more polite though if you say 私は１７歳です。
if someone ask me 「おいくつですか？」or「何歳ですか？」, I would say 「～歳です。」
as you can see,we tend not to say わたしはor あなたは in general conversations.

Answer

You would say:私は十七歳です。