Hey there! Thie picture above is done with little to no guidelines or draft, so I'm sorry if it's rough. This picture is mainly to demonstrate the use of female, age-appropriate guidelines. Although this post is meant for new artists, anyone can read!

First off, let's think about ages and what their bodies are going to be like. I've created a diagram of each body type and corresponding age range below. After we have a good look at that, we'll go over each body type and age one by one.

Young child

As you see in the above picture, the young child's body type is a pear like shape. At the age range of five to seven, children still contain some of their baby fat giving them a rounder torso shape. As they get to ages eight to ten, the baby fat is usually completely eliminated.

As you see, I've added details to the body, but you can still see the pear shaped base. The chest slightly rolling over the stomach due to baby fat, and the stomach connecting to the leg in one swift, smooth line.

Here I inked the drawing in so you could see more clearly.

Tweens

Tweens can be ages ten to twelve. This age is usually right before puberty hits, or in the very beginning stages of puberty. Usually the chest is still completely flat, but sometimes young girls do start developing earlier than others. To create a medium between the two chest shapes, I'm going to draw the chest at an ever so slight curve.

And now to ink it in to make the lines clearer.

Young teens

At ages thirteen to fourteen the girl has either already hit puberty, or is in the middle of hitting puberty. With these facts I decided to draw her chest not completely developed but very noticable. Also note that in manga and anime, the female character's chests are often oversized.

And now to clear it up.

Older teen

At ages fifteen to eighteen, girls are usually completely developed and the only difference between their bodies and adult bodies is torso size. I've found that the younger you are the shorter the space between chest and hips will be.

And now for the clear up.

This should also be the appropriate body type for your steriotypical teenage anime waifu.

Adult

Now our girl is completely developed, matured, and and her torso has lengthened. Assuming that she has been exercising and eating healthy, this should be your body type, and classic anime mom-bod.

And now the clear up.

Anime is so unrealistic.

Putting our bodies to good use

Now we have our basics, we can now turn our body types into characters. Don't come for me in the comments over my quick sketches, because that's exactly what they are, quick sketches.

To demonstrate, I'll draw a character of mine, Layla, at diffrent ages.

Age six:

Age eleven:

Age fourteen:

Age seventeen:

Age twenty-five:

I hope this helped you out some! I also have another tutorial in the works on giving your manga girls more realistic body types, so stay tuned!