What do you think the age of consent should be?

A 25 year old acquaintance of mine is dating a 16 year old girl who is about to turn 17. Now this is legal but it still seems a bit wrong to me. I don't understand his attraction to a 16 year old. How much can they have in common? Is she old enough to know what she is getting into? In Spain the age on consent is 13!!! Whereas some states in the US have it set at 18. What is your opinion on the matter?