When the death rate for old people and the infant mortality rate are high, with no migration, there will be in the population a higher number of

This question requires you to reason about how simultaneous high death rates at the extremes of the age spectrum affect the population's age structure, assuming no migration.

When the death rate for old people is high, the elderly population shrinks rapidly. When the infant mortality rate is also high, many newborns and very young children die before reaching older childhood. With no migration to alter the numbers, the people who survive in largest numbers are those in between these two vulnerable groups - the younger people (broadly, adolescents and young adults who have passed the dangerous infant years but have not yet reached old age).

The result is a population structure with a bulge in the younger working-age bracket. Children are fewer because many die in infancy, and old people are fewer because of their high death rate. The population therefore has a higher proportion of younger people.

Note that "younger people" here is distinct from "children" - children include infants whose numbers are being reduced by the high infant mortality rate.