The game's age rating does not necessarily make the stream's rating, but the streamer themselves definitely do. A streamer like Nordenheim might explore the E-rated aspects of an M-rated game, such as The Elder Scrolls Online, while ensuring his commentary and chat are as family-friendly as possible; meanwhile, another streamer might play a game with a lower age rating, such as Minecraft, and not manage their language while openly making crude jokes and discussing mature topics.
The game's age rating does not necessarily make the stream's rating, but the streamer themselves definitely do. A streamer like Nordenheim might explore the E-rated aspects of an M-rated game, such as The Elder Scrolls Online, while ensuring his commentary and chat are as family-friendly as possible; meanwhile, another streamer might play a game with a lower age rating, such as Minecraft, and not manage their language while openly making crude jokes and discussing mature topics.