In the end the decision is DCLs and not posters with strong opinions here.
On RCCL there Palo equivalent allowed 14> 18 year old in,mew did it, it was no problem at all. The rules were they must go with adults and they were allowed to book from 6 pm, to 7.30pm, not later.
My children went and enjoyed it and no cruisers there had any problem with it at all. It didn't lower the tone, it was not any slippery slope. If adults wanted to be adults only then, they could book post 7.30 pm.
On
DCL my son is 17 and three weeks off 18 he cant go to Palo that's the rules I respect that, but he want to the adults restaurant on RCCL 4 years ago. They were respectful enjoyed it, and no one had any problem with it.
As said, it's DCL rules, smart jeans can be very smart, I have no issue with it, it's their sandpit, their rules, some should remember that. DCL is run by its management, and contrary to belief, not an Internet forum.