1
first experience (of something)
「カイちゃんよー、お姉ちゃんはスゴク悲しいよー。こんなことで人生初体験だよ」「え？」「夜・逃・げ」
"Kai dear! I'm really sad! Having my first experience in this way." "Eh?" "Skip-ping-town."
2
first sexual experience
初体験の相手は、年上の女性だった。
My first sexual experience was with a woman some years older than I was.