Can we prove the word fornication actually means pre-marital sex in the bible?

I've also heard that it meant, sex with many partners or adultery. Also people didn't live very long back then so, they got married very young. Am I right maybe 14 or 13 or perhaps younger? I'm not really sure how to apply that to todays life, and sexuality as a christian. Should we get married younger. You know the older we are the more our bodies have the innate desire to pro-create. My cousin got married young and she's pretty happy and very christian she saved it for marriage. I don't really know how to feel about all of this.