It is not possible to have consensual sex with a minor.
Yes, it is if the other kid is one too. The law differs by state and the rules change according to the crime. That society is arbitrary in its handling of crimes should tell you something. Underage sex is wrong for a whole host of reasons but you have to consider that a difference in age of of two young people that could be as little as 14-15 months is negligable.
At one time or another plenty of people were getting married at the age of these two kids. Different era perhaps but I guess society changed its mind on what is legal and proper.
Yes it is.
In New Hamphsire the age of consent is 16. If the girl were 16 or even 17 she could legally have consenual sex and still be legally a minor.