How do we know who is meant for us?

I  wonder why when girls at the age of 15 – 20 meets or want to meet a guy. They want him to be tall,dark and handsome with a lot of swag and don’t care about money ,fame or power .Then in their early twenties its more of  being in love, then mid twenties, they still want those things but its more of if he’s good in bed or if both of you are compatible in bed. Of course we all know finding a partner is more than that, but that seem to be the first thing that pops-up in our head when we meet him for the first time.

Therefore do girls/ladies that age really know what they want? is there a particular age to know who we are supposed to spend the rest of our lives with? do we take risk? It actually works for some people that get married at a young age but I’ll bet its not easy because they learn at the course of time, especially if those people are just met to be together.
At some point (late twenties and beyond),some ladies don’t care anymore. They just want to start a family;as long as the guy cares for them and respects them that’s just all they need.

On the other hand, there are people who we refer to sometimes as “old soul”, these are the people that know what they want at every stage in their life, even while they make mistakes sometimes,they still know what they want and go for it. whether its career-wise or relationship-wise; these are the people that get lucky sometimes.
The question is do we really know who is meant for us by just sighting them,meeting them or spending years with them? how do we fall in love? do we have reasons why we get married? do we marry for love or what? how do we define love at first sight? can one really love at first sight or just in love with the sight of the person? i guess we’ll never know, we just have to live through it.