0 - 10 is a pretty horrible scale. If I was rating myself on such a scale I would have got lower as I got older. At 21 you can be a "10" by being the best in your CS class. At 35 you have to compare yourself to people with serious achievements.
No, the same standards apply regardless of how old you are. If this question comes up in an interview, you are being asked to rate yourself on an objective scale, not on a curve based on where you're at in your career. A 21-year old (or 35-year old, for that matter) who rates himself a 10 is almost certainly way overconfident and will run into (or cause) all sorts of problems. What you're complaining about is actually one of the main things this question is trying to determine.
Put another way, someone who rates himself a 10 because he's the best in his CS class is someone who doesn't know what he doesn't know, and is also likely to be a primadonna. Someone who shows some thoughtfulness, skepticism, and humility when answering this question is more mature and self-aware, and is much more likely to be an effective part of the team. The rest of the interview is supposed to establish how much they actually know.
But not knowing what you don't know is really to be expected of people who are recent grads in particular. I think this reveals why the question is problematic unless it is worded very specifically.
A 10 from a recent grad could legitimately mean "I've had no problems learning everything that has been thrown at me so far and have very high confidence that I will easily be able to learn more" as much as it could mean "I know everything".
Then we will have communication issues and it will be difficult to work with you. The question was worded "On a scale of 1 to 10 how fluent do you feel in X". This is explicitly -fluency- (or phrased another way, "how well do you know X"), not something completely undefined like "How good are you at X?". The question is asking you to project your own perceived scale over a reasonably definable axis, not an ambiguous one.
Project the question into another domain - "I passed my intro Spanish class", "Oh yeah, on a scale of 1 to 10 how fluent are you in Spanish?", "I'm a 10". Nope.
There's a difference between being fluent in something and mastering it.
You might be capable of reading and writing Java code, but that says nothing of your knowledge of JVM internals or of any particular framework etc.
It's like the difference between being fluent in Spanish and being able to use the language to write moving poetry or having a very extensive vocabulary.
I think it is much better to be as specific as possible about what skills you expect the candidate to have going in and what you expect them to be able to learn on the job.
