When you look at the risk curves, in fact the risk with blood alcohol levels between 0.05 and 0.08 is not very high. The risk really increases beyond 0.08, and then it goes up exponentially.
The answer about young drivers is that, drunk or not, they have many more accidents, and the probability of being injured and dying when you're in a car with them is much higher. So we need to be able to understand the distinction between risk-taking, which is increased with blood alcohol level but is there in youth, and blood alcohol levels that are very high, which in and of themselves then create an immense risk for accidents, and it really goes high.