Great, then you're enforcing rules on one group and not the rest. They're not necessarily even the problem group. Bullies and snowflakes are often the adults. And why 18? Why not 16? or 21? Are you going to make people prove their ages in order to take part? If not, you're going to just get people lying so they can either be protected by the CoC, or lying so they can go around being an asshat. Are you allowed to be an asshat to the people under 18? If not, then you've just created a CoC.

There are just too many issues with having a CoC for kids and not everyone else. It doesn't solve anything and makes it even harder to figure out interactions. You're better off with a simple CoC for everyone, or none at all, but don't pretend that you're community is adults only so you can justify allowing people to be crap to each other.