I feel like this question is based in the assumption that the age requirement change was arbitrary. However, it wasn't arbitrary, to my knowledge: It very specifically restricted site membership to the age of legal adulthood in the United States, where the group is incorporated, or required...
I definitely put myself at risk plenty of times without realizing it when I was a minor, back in the early days of the Web and on AOL and BBSes before that. I am simply lucky that nothing bad ever happened to me. I also suspect that things have only gotten worse and more dangerous over the years...
If I had to guess, I would also suspect that some aspects of the current political climate in the US, where the group is incorporated/administered, make it especially important to avoid any chance that someone could accuse the group of encouraging improper interactions between minors and adults...