Do you have any skip, display, or branching logic set up already? If so, can you show/tell me more about how it is set up?

I would actually remove your custom validation from the question, and instead set up skip or branch logic that ends the survey for anyone who is under 18.
One option is to set up skip logic so that anyone who answers 1-17 gets skipped to the end of the survey. Do that by clicking on the settings wheel for the questions, clicking "add skip logic" and then setting up your logic so that those who select 1, 2, 3, ... 17 get skipped to the end of the survey. Here's an example in the screenshot below. In my example, instead of writing out the numbers 1, 2, 3, ... 17 in my question, I combined them into one option called "17 or under."

Alternatively, you can add branches in your survey flow. Go to your survey flow, click "Add a new element here" and then select "Branch." Within that branch, add a condition that will trigger the branch if the respondent answers the age question with any number under 17 (or in my case, the response option "17 or under"). We want the branch for respondents 17 or under will take them to the end of the survey. To do that, add a new element underneath the branch by clicking "Add a new element here" and selecting the red "End of survey" option. See the screenshot below what that looks like once completed.

Branching logic will work best if your age question is the last question in its block. You can also use a combination of skip & branching logic if there are other questions you want those aged 17 or under to see.
You can create a custom end of survey message to show to participants who are 17 or under that tells them they need to be 18+ to participate. To do that within your survey flow, click on "Customize" in the red "End of Survey" element. A pop-up will appear, like in the screenshot below. Check to override survey options, then select the bubble for "Custom end of survey message" and create or select your message.

I used the first options and it worked! Many thanks!!