Population age

ive read in documents about population death after you get to a certain amount of people in your country. for example today I noticed that I am now having more deaths than births. is there any solution to this problem?? what should the average age look like and how do you fix this?

https://www.simcountry.com/discus/messages/16295/25416.html?1451965914

This is the guide that I have been studying to learn how to prevent the problem you seem to be experiencing.