During medieval period ( w/c the time frame the story happens) it is natural to want to have a child at that age because men are always called for war and you never know if they will comeback alive.
What was said above
Also, people died much younger in those times disease and whether also lead to lots of loss of life and stuff, so they needed to make children at young age because of it.
the average lifespan for people around medieval/middle ages was 30. If you were rich, maybe 40-50...so yeah.
If you didn't give birth sooner, you wouldve just simply died in a few decades due to various factors.
but two or three kids at 18years old is their normal expectations???!!! (⊙…⊙ ) i would have understand if they're panicking for heir if she's 28 (though even this is outrageous). but at 18?! (⊙…⊙ )