Egregious CSAM aside, none of these websites, including sites like Reddit, can be 100% confident that the people posting images are doing so of people who are over 18. For example, how could Lemmy or Reddit be confident that (insert gonewild poster here) is over the age of 18? As far as I'm aware the only checks that they do are to confirm that the poster is the person they say they are, not that they are of an appropriate age to post.