For one, their psychosocial level is still around the same level as an 11 year old until about the age of 18. The brain is not fully developed until around the age of 25.
People call them pedophiles because they don't know any better or they simply exaggerate in order to show their disgust.
Menstruation does not mean that a girl should be legally allowed to have sex with anybody let alone a 20, 25, 30yr old she is still a kid and in our society we try very hard to protect our children from both emotional and physical harm.
I would leave the age of adulthood at 18 and/or change it to 18 (some states I believe are slightly different)
Edit: This is a response to the OP, apparently I'm a slow typer.