When do they get to 18kg?

Okay don't know where to put this or if it is a silly question but i'm new at this so here it goes.

I know i am only a lil way along but i am one of those really organised people. I'm not planning to buy anything for another 3 months or so but i want to strat looking around for good prices etc etc.

Most prams/car seats say newborn to either 18kg or 20kg.

How long until most babies get to this weight? It doesn't seem like a lot but then again like i said i'm new at this.

around 2 years, or even 3

My daughter is 3.5 years old and weighs 10kg,so we'll get good use out of her car seat.

My DD is jus t16 months and she is olny 10 kg still its very hard to tell untill bub is growing ect