One more thought - many posters have said it's a matter of trust. I don't agree. I trusted my daughters and they have always proven themselves worthy of that trust.
However, I can't be sure I can trust others that they are around so I was careful that they didn't end in situations where that might be a problem. That wouldn't be my primary reason for saying no to this situation, however.
It would be because I think that each age brings situations/events that are appropriate for that age. Every parent has their own personal yardstick on what is appropriate at what age. There are parents who think things are appropriate at younger ages than I did and that was fine for their kids. But for mine, they were required to stick my yardstick of appropriate, and I wasn't budged by what other parents thought or did. (The old 'everyone else is doing it' speech).
So you will get a million answers if you ask a million people but at the end of the day, use your own yardstick. It's the only way.