Look, "Kid" can be either an insult, or an endearment. We call our married children "Kids" - "Where are the kids?" or "The Kids would like this," or "The kids are spending Christmas with their in-laws" - It's a familiar term of endearment, even though our son is 30.
I might also say "Kiddo" - "Hey kiddo, watch your step..." To someone younger than me, but that would almost be an insult. "Look, kid - you don't know what the hell you're taking about." - to a thirty year old man, would be almost as bad as saying "BOY" - it's an insult to manhood.
But I've had talks with students still in high school - 16, 18 years old. "You're young adults in some ways, but in some ways you're still kids," - and they get that.
Personally, as someone in their fifties, if I was NOT using the word as a term of endearment, or joking with someone I wasn't already familiar with, I'd stop using the word "Kid" as a form of address - by their early teens. At which point, I'd switch to "Young man, young lady"
If you're speaking of 'kids' in the generic sense - then it ends when they are of legal age. "Kids in high school..." can be heard every night on the newscasts. "Kids still in college" - a little less so. "Kids" is a substitute word for children. "You can keep your kids on your insurance plan, until they reach 26" "My kids taught their kids how to surf..." It just means 'children' - as in relations.