And, if your sexual activities started when she was 16, you are going to prison.
The clock doesn't start now.
By the way, let's pretend for a moment that you did start sleeping with the child when she was 16.
You do realize, of course, that you are on the hook for the next several years, right? That all she has to do is pick up the phone and say, "yea, I didn't know it at the time, but this guy had sex with me at 16 and I want to testify."
How does it feel to know you have put your future in the hands of a child? I wouldn't hack her off if I was you. Ever.