"Good customer service" to one person (in making exceptions to the rules) may mean poor customer service to others. If
DCL makes an exception to letting a non-adult into the adult area, that may disrupt others who wish the adult areas to be truly adults only. So for them, it's poor customer service. And as
@PrincessShmoo pointed out, if DCL allows exceptions for one situation, how do they then draw the line at making other exceptions? Yes, you knew your teen was mature, but DCL doesn't know that. And they can't guarantee that other close-to-18-year-olds will also be mature. So they make a policy and they stick to it.
I certainly understand how this impacted your cruise, and I'm not trying to minimize that. But, that is also a limited circumstance that is not likely to happen for you again. And there are plenty of areas on the ship where everyone can hang out together, regardless of age. I understand your frustration but I don't think the solution is for DCL to make exceptions to the rules on who's allowed in the adult areas.