What's the appropriate age for a Facebook account?

Facebook has it pluses and minuses, I suppose.  On the one hand, it provides a spot for kids to connect.  On the other hand, it's just one more thing as parents, we have to monitor.  And yes, as with everything else online, there are potential dangers lurking.  That said, I suppose if I were a 'Friend' of my child, I could at least monitor what's going on, while still allowing some 'freedom.'  My question is: "What's the appropriate age to have a Facebook Account?"  Once they're a teen, is that old enough?  Should I 'demand' to be accepted as a 'Friend'?  What's your opinion?