eli5 If children’s (0 to 5) immune system is weaker or still growing, then why is it ok to let them get sick to build their immunity?

Question

I’m hearing lot of moms say that it’s ok to let children play in dirt, attend daycare with sick kids, not sanitize the living spaces too much, put dirty stuff in their mouths because they can only build their immune strength by getting sick first(?!).
I have read the hygiene hypothesis paper and many research papers that have disputed that paper aswell so I’m just curious to learn the science behind the idea ‘let them get sick first’.

In: 1

Answer 1

There are certain illnesses that have been around for centuries where it’s better to get it younger. Like if you get it below age 7 you’ll definitely be fine, and if you get it after 15 you have a much higher chance of dying. And you can only get it once so it’s better to get it younger.

People just sort of assume that applies to everything when it really doesn’t.