It would be almost trivial for us to talk about the differences in race, age and sex as there really is no standard. In general, older people have longer ears and noses, thinner lips, etc, but not always. The same goes for differences in race and sex, though a broad enough study will prove these differences can be observed from person to person, regardless of sex, age, race or any other form of arbitrary categorization.
Of course, we can perceive certain features that indicate these differences, representing these in our designs but it's quite important to focus on the character and story rather than sex, age, race category. Draw the hopes of a hero, draw the malice of a villain, rather than reducing them to a categorical expression.
To develop your skills in depicting a variety of characteristics, try drawing from people you see around you. Train stations and airports are great places for this. You'll be surrounded by people from a variety of places with all sorts of bodies, heads and faces.
Though it's fine on occasion, try to avoid beauty magazines, as models tend to have standardized features and your study will become stunted as a result.