Regardless of your Home Depot example (which I happen to agree with, by the way), your assertion that "kid" means someone who is under the age of 18 is utterly ridiculous to those of us who actually have kids.
I agree with you on the parent will always refer to a child as "their kid" no matter the age. But I also understand their point by saying if your above the minor age child makes a grown up decision and gets hurt it is ridiculous to say your going to pay the business a visit on their behlaf.