The concept of rape is already enough to do justice.
All juries already agree that young children cannot consent to sex, and that this inability becomes less clear as the child becomes an adult. A rape charge alone is enough to convict someone of sex with children.
This solution would neatly account for the real-world gray area between childhood and adulthood.
Why does this idea matter?
By using a cut-off point., we've developed a sort of knee-jerk tradition whereby a 15-year-old's consent is considered totally meaningless(like a non-fact) and offender damned to hell as if he raped a toddler.