This is correct, not sure why it was downvoted. I think a major reason why people did not get very sick from poliovirus when very young is because of maternal I

This is correct, not sure why it was downvoted. I think a major reason why people did not get very sick from poliovirus when very young is because of maternal IgG passed through the placenta before birth, and maternal IgA (edit, sorry, probably IgG through breast milk as well) passed through breast milk. This only lasts for six months and even by then it's waning, so children should get vaccinated with trivalent eIPV (enhanced Inactivated Polio Vaccine) as soon as their immune system is developed enough, probably by four months of age in enough cases to start giving the first dose. Some countries can't handle the cost and logistics of giving safe injections of eIPV, but you can't give multiple serotypes of OPV (Oral Polio Vaccine) at the same time because there is a dominance hierarchy that interferes with generating immunity against all serotypes, so at least one dose of eIPV should be used before starting OPV if at all possible. Polio has three serotypes without long-term protective cross reactivity, so you can't rely on natural infection to prevent future disease (possibly as a teenager or adult) caused by a different serotype.