I wouldn't worry about the demographics too much. They contain very little useful information.
You might have lower population than the AI, but your smaller population can work good tiles with good tile improvements and invest the tile yield into useful things.
Likewise you might have lower life expectancy than the AI, but as long as your cities aren't unhealthy, they don't actually benefit from extra

.
Or the AI could have a lot more soldiers than you, but they're gonna be scattered around in stupid locations whereas you have the most useful types of units at the time when you really need them and composed in stacks that can actually achieve something in a war.
This changes especially as you climb levels and the AI gets more and more bonusses towards everything. They can settle a totally useless city with no resources and still grow it fairly big because of the bonusses they get.
Anyways, some general guidelines about food and population that apply to most games:
-Food is king.
-Settle new cities primarily if there is a good food tile (or more) in the Big Fat Cross.
-Especially in the early game and if you're not playing a Creative leader, make sure the food is in the first ring so you can work it immediately.
-Generally, improve food tiles first.
-When settling a new city, have workers or workboats ready to immediately improve the food.
-Focus early game research on food tiles available so that you can improve those immediately.
I could go on but for more detailed advice it would be good to know what level you're playing on and screenshots or a savefile of your current game.
Edit: before I forget: The AI never uses Slavery to its full potential. As a human player ideally you wanna whip a lot more than the AI especially in the early and midgame, which automatically results in lower population than the AI. Yet it's still the most ideal way to play.