Complex sentence: Any date from January 1,1950 to December 31,1980 is valid: otherwise the player's character would be too old (causing the game to end prematurely) or too young (causing the game to unnecessarily strain itself to accompany non-existent futuristic dates).
Split into simpler sentences: Any date from January 1,1950 to December 31,1980 is valid. Otherwise, the player's character would either be too old (prematurely ending the game) or too young (causing the game to unnecessarily strain itself to accompany non-existent futuristic dates).