Brackets of 50 years are a good objective way to count things. 
Early is the first 50 years. You can't challenge absolutely all the space creeps, the galaxy is pretty unexplored, there are many planets to go around to colonize. Also is pre-Battleship era.
Mid-game is the next 50 years. Usually that point where almost all the borders are drawn. You start to see government changes, and the neutral space creeps like Drones and Amoobas are gone. Terraforming/Uplifting begin to happen.
Late is 100 years and beyond, when Fallen Empires awaken and Crisis can happen!
Early is the first 50 years. You can't challenge absolutely all the space creeps, the galaxy is pretty unexplored, there are many planets to go around to colonize. Also is pre-Battleship era.
Mid-game is the next 50 years. Usually that point where almost all the borders are drawn. You start to see government changes, and the neutral space creeps like Drones and Amoobas are gone. Terraforming/Uplifting begin to happen.
Late is 100 years and beyond, when Fallen Empires awaken and Crisis can happen!