If you make the head (A) roughly the same size as the torso (B) you will end up with a cute-looking character. You can vary this head:torso ratio if you want to create a specific type of figure, but this basic rule applies if you want to create cute human or animal characters.
When creating a human-like character, take the modelled head or torso as a unit of measurement: a good height for a person is five or six heads high, the legs are two-and-a-half heads long, and the arms should fall slightly higher than the knees.
I would always encourage you to discover your own cute proportions for modelling and to use the ones suggested here as a starting point. As well as experimenting with different head:torso ratios you can make the legs, arms or neck longer to stylize the character.
You can portray different ages simply by moving the eyes. Once you have made the head, draw an imaginary line across the middle of the face. To make a young person, place the eyes below this line.
For an older person place the eyes on or above the imaginary middle line. As a rule the higher the eyes are above the line the older the character looks.
The eyebrows are part of an imaginary circle that sits around the eye: this imaginary circle can be stretched or squashed to achieve different expressions.
The height of the ears should be between the eyes and the base of the nose.