I'm sure some of them were. Young teenagers are the main group that goes nuts over celebrities.
That simply means that the 'celebrity' has even more of a duty to ensure that they don't take advantage of that. It's not a reason to say oh well then, what could they do, they thought they were old enough...
We have 'think 25' for alcohol. Is is really so unreasonable to keep something similar in your mind if you are a famous person and you know that young people idolise you and that they may only be 14 or 15? Not to ask yourself does this person look 16, but to ask yourself does this person look 18, or 20 or 21.
And not only how old do they look but how are they acting. Are they acting like an adult who is capable of making this choice? Or are they a giggly kid?
Should you ask for a birth certificate? You should certainly be very sure that you are having sex with an adult and not a giggling kid who's totally star struck.
I can certainly tell a 14 year old from a 20 year old by the way they talk, walk, hold themselves and the things they choose to talk about. I wonder why these people can't.
And, you know, you don't HAVE to shag someone just because they're there, if you're not totally sure that they are old enough (or sober enough)