Sure. It may be more commonly used by older people, but it would certainly be understood by just about everyone.

I've never used it, but I understand it. Context explains its meaning so it will be understood immediately even by people who don't know its meaning. If it's bedtime, everybody is tired, and you say "I think it's time to turn in" then what else could it possibly mean?

Yes, the phrase "turn in" is still used to refer to the action of going to bed, though it might sound a bit old-fashioned or formal in some contexts. It's more common in conversational English, especially among older generations, but it’s still understood and occasionally used by younger people as well. For example: "I'm going to turn in early tonight." "After a long day, he decided to turn in." In more casual conversations, people might say "go to bed," "hit the sack," or "call it a night," but "turn in" remains a perfectly acceptable and recognizable expression.

Hi! Yes, the phrase "turn in" is still used to mean "go to bed," though it’s somewhat old-fashioned or informal. People still use it, but more commonly, you'll hear phrases like "go to bed" or "call it a night."