Well it's a matter of anatomical dimensions, and there's so much variation therein that's there's probably a feasible amount of wiggle room for his argument to fit in between....
...but for all practical purposes, I agree with what you're saying.
What's more important though is the matter of understanding and appreciation of sexual activities (which is why naivete is such an important characteristic). Sexual development doesn't occur until puberty, so while there might be oddball situations of youngsters growing sexually, the law is meant to preserve general cultural attitudes, attitudes which would reflect how the overwhelming majority of children do not develop as such.
I mean if a kid doesn't have a taste for sex, why could he give consent unless intimidated; if he performs sexual activities as a matter of commercial exchange, how doesn't that fly in the face of the wage-slavery argument (nevermind concerns over sexual trafficking and prostitution)?
Age might be an arbitrary number in itself, but it is considered a reliable indicator of maturity (both mental and physical) so to dismiss it without a comprehensive alternative seems extraordinarily reckless.
