Step-by-step: Develop distinct facial types
When drawing a young, healthy character, knowledge of the basics of proportion are essential. I hint a little at the underlying muscle and bone structure, but primarily aim to achieve balance and subtlety. Too much of any one specific characteristic can be jarring for the viewer.
A fuller face can be a challenge because the bone structure and musculature are obscured by fatty masses, so anatomical diagrams may fall short. I use a fair bit of photo reference here to study how the fat around the jaw and cheeks alter the basic forms of the face.
For a wizened or gaunt face I exaggerate the bone and muscle structures, making use of anatomy diagrams as reference material. If I want to age the figure, looser skin around the jaw and neck and bags under the eyes help to lend authenticity to more obvious wrinkles.