noun, noun which may take the genitive case particle 'no'
first time in one's life
Used to describe an experience or event that is happening for the very first time in a person's life. Often used attributively with の or as a noun.
人生初の海外旅行に行く。
I'm going on my first overseas trip ever.
人生初の経験だった。
It was a first-in-a-lifetime experience.