The thing is, mortality rates are a lie. Why? Because they are based on the average of the whole population. What this means is, if you have a high child birth morality rate but your adults live to 100, you'll still have a low mortality rate. Russia has a 16.5 Infant mortality rate and it is a shameful fact for the country. But medication is improving and so is education (something that was lacking under the Soviets IRT birthing, etc) so the mortality rate will do a sharp increase over the next 5 years.
I am not sure how you arrived at this conclusion. Mortality rate of a group is the number of deaths in a given period of time, typically one year, divided by the size of the group. It matters not how other parameters of the group, such as age, are distributed.
Further, infant mortality rate is defined as the probability of dying between birth and one year of age. This too has nothing to do with the age of adults.