@Riger I think you are misunderstanding what's happened. It might be common knowledge that kids are supposed to get mana before the age of 4-5. The only one that knows that only reincarnators don't get mana is Koron. So peasants should know that kids get the mana spot between the age of 1 and 4.
@Riger
They already explained it, you're supposed to get your mana spot between the age of an infant to 4 years old. Ash is 5 years old in chapter 1 and he has no mana spot.
It's probably common knowledge that you are supposed to get your mana spot before the age of 5.