[markmark]

It's harder to do studies that seperate out adults who didn't have fluoridated water as children, but for example from https://pubmed.ncbi.nlm.nih.gov/23456704/ "In this nationally representative sample of Australian adults, caries-preventive effects of water fluoridation were at least as great in adults born before widespread implementation of fluoridation as after widespread implementation of fluoridation."

So if you didn't have fluoridated water as a kid, having it as an adult still provides a benefit. This particular study doesn't say anything about having it as a kid and then stopping, but again, it's hard to find significant populations that have experienced that.