I don't think this is much of an issue at all. The path of least resistance, by an overwhelmingly wide margin, is just using a proxy, TOR, or whatever else to bypass the filtering. Sites will be doing the bare minimum for legal compliance, and so it won't be particularly difficult.
Beyond that I'd also add that for those of us that were children during the early days of the internet, "we" were always one click away from just about anything you could imagine in newsgroups, IRC, and so on. It never really seems to have had much of any negative effect, let alone when contrasted against the overwhelmingly negative effect of social media.
I don't really know why that is, and I half suspect nobody really does. You can come up with lots of clever hypotheses that are all probably at least partially true, but on a fundamental level it's quite surprising how destructive 'everybody' communicating online turned out to be. And that obviously doesn't end just because somebody turns 18.