I have done. My daughter wrote in a diary when she was about 13 - so it's 10 years old now. I read it a few years ago, when she'd left home and it was left in her bedroom. It was full of school, 2 particular boys she fancied, crying, falling out with friends, making up with friends. I told her I'd read it, she didn't mind. It was very typically teenagerish.
When both of mine were teenagers, I always had a look about when I was "cleaning" their rooms, read many a "note" - you know, those notes that get passed on in class and put in pencil cases.
Mine didn't have mobile 'phones or the computer when they were younger, they weren't commonplace then as they are now, but if they had of had them, I would of checked them too. I like to know, lol. What a good question.