My experience has been that people get to a "good enough" level in a second language and their motivation for working hard goes down dramatically. Children, on the other hand, have a lot of incentive to work hard on their language every day. If you've ever known a child with a speech impediment, you can see how tortuous it is to speak differently than your peers.
The other thing that colours my judgement is that every person I've ever met that has tried consistently to remove their accent has succeeded. However it's a 365-day per year, multi-year project. I've met very few people willing to undergo that experience.
Citation?
This doesn’t match my experience: I have heard at least a few hundred non-native speakers of my native language (Russian), and every single one of them had an accent, obvious after just a few words. The few exceptions were be native speakers of other slavic languages, but even then I suspect they learned Russian as kids.
The jury is still out on that. See https://en.wikipedia.org/wiki/Critical_period#Linguistics.