I was thinking in terms of updates to the game as well as how gameplay progresses for an individual playthrough.
When I first started playing 1.3, after the community center and theater you'd be late game. So I still kinda consider Ginger Island late game. It was added late!
Early game could be the whole time you're working on the community center, but I've deliberately slowed my gameplay a few times. So I might have a fully thriving farm, maxed friendships, full museum, and lots of money, but still need to collect a few items, maybe as late as year 3.
I used to feel like the theater, level 100 skull cavern quest, magic ink and witch swamp were late game. Since 1.5 that's shifted. Now I feel like, the early part of Ginger Island is still mid game- opening the walnut room and getting to the forge is when I say, this is late game.