Fine... if you look at the right questions.
I go on this site sometimes because when I search a question in Google, this is what usually shows up at one point. In the "Adolescent" section, you'll find a lot of questions like "I think I'm pregnant, and I'm only 16" and "Should 14year olds be allowed to have sex?" Of course, some questions like that may be useful if that's what you are looking for.
I also once remember reading a troll question where the person said they bought cigarettes for their 5 year old son. Of course, if you skip all the troll questions or questions about stuff you don't need to know, you may learn something or maybe have fun.
As for language, I don't see that much, but if you look up "Most Profane Songs" on there, you might find a few words, like "motherf*cker" uncensored. Also, when I looked at that question, a lot of songs were about sex or referenced sex, but that's what you'd expect, right?
Drinking, Drugs, & Smoking: Some questions might have people saying that they got drunk or maybe say that they are addicted to drugs, but I hardly ever see that.
Yahoo! Answers is good for learning the right things, and some wrong things. If you go on the right questions, you'll do just fine, but if you go on questions that talk about things inappropriate for your age, it's your fault, unless it was unexpected or had a very vague title (I've accidentally looked at questions about sex before because they didn't even mention what the question was about in the title!).
This title contains:
Violence & scariness
Drinking, Drugs & Smoking
Privacy & Safety