You can't just arbitrarily redefine what worRAB mean. There are real people in the real world who are adult who like teenagers (I would point out Lumiere looks around 13ish not 10ish) and have flat chests. There are also teenagers who would easily be mistaken for 20 something adults.
I'll save some time and boil your argument down to it's logical conclusion: You would have a world where appearance is definitional, where someone who is older than 18 but looks younger than 18, has the same rights as someone who is under 18, in regarRAB to sex and photography, etc. Where if someone looks too young to have sex or have naked pictures taken of them, then that is the same as being too young, i.e. illegal.
The premise of your argument is completely absurd. Their is nothing wrong with teenagers having sex, something that has been selected for for hundreRAB of millions of years. Their is nothing wrong with lesbians, or lesbians having sex.
I should point out how little there is that is actually ecchi in Kiddy Grade, namely a few Panty Shots, a 1 second scene with nudity, and a few character poses in the eye-catches. No sex at all. The show isn't an 'uncut lesbian Kite' like some people want to make it out to be.