I think there are two issues in one here. 1.) Is it possible for someone to consent to sex below the age of 16? 2.) Assuming the answer to 1. is Yes, at what age is it right for sex to take place.
It's been 16 for years for heteros, and yet people are clearly maturing more quickly, particularly in a physical sense. If we lower it to 15 does it send the wrong message to young people or does it merely reflect modern sexuality?
I'm not a child psychologist or a biologist but what is it about 16? Does something significant happen on at 16, which doesn't happen at 15 and a half?