The business requirment that is communicated with the BA/Consultant: A legitimate user needs to be between the age of 18 and 56.
Developer should meet the required functional requiment : Implement or reuse program logic that will validate a provided numeric input agaist a given range. eg: IF 19 within the range of 18 to 25 THEN PASS ELSE FAIL
Test scenarios should be written to: Test both positive and negative test cases to validate the business requiments.