What should an eight-year-old be able to do in terms of development?
I can broadly sketch for you the skills that an 'average' eight-year old possesses, but please keep in mind that development is a variable process and the fact that one eight-year old can hit home runs while another can't hit at all doesn't mean that either are abnormal. If you are just curious about general skills, here are a few:
Fine motor: Prints name, holds pencil well, shouldn't mix up many letters, draws a person with many small details.
Gross motor: Can ride a two-wheel bike, climbs ladders, pours a drink. Most eight-year olds are coordinated enough to join an organized sport like soccer or Little League.
Cognitive: An eight-year old should be able to learn how to tell time, add two columns of numbers, repeat five digits just recited to him, read a simple paragraph and comprehend the main message in it. His language should be fully intelligible and almost always grammatically correct.
Usually, when we are concerned about the development of a school-aged child, it is in terms of achievement. If your concerns are in the academic realm, work with your school to elucidate the specific problems. If you are worried, consider asking your pediatrician for a referral to a developmentalist.