「まだ○年しか生きてない」は、
I'm only ○ (years old). でどうでしょうか?
まだ〇年しか生きてないから、人生で最大の困難な体験は?って聞かれても知れてるよ。
I'm only ○ (years old), so such a question as "What is the most difficult experience in my life", is hard to answer. I only have so many experiences to use as an example.