Personally, I think you've missed the point. The people behind SOL aren't saying that you can't or even shouldn't write about younger people having sex. They've never said that.
What they have said was that you can't post such stories on their website. That's not a subject for argument. It's their website. They can make whatever rules they want. They can prohibit stories that mention Toyota vehicles if they want. You can still write car stories, but if you mention Toyotas you have to post them elsewhere. Follow their rules or go elsewhere.
In this case, they have posted in various places that they have a rule, they have explained that rule, and they have even given their reason for that rule. Whether you or I or anyone else believes that their reason makes sense or is true or relevant does not matter. Personally, I think that the rule is silly, but this is their house. All that matters is that this is their website and they have posted rules for its use. Follow the rules and use the website, or don't.
When you complain about the rules on someone else's website being wrong or unfair, you are a religious bigot insisting that others follow YOUR values. If you came to my house and started telling me that I'm using the living room wrong, I'd throw you out on the curb.
Have you considered starting your own free web-publishing website that has whatever rules you want? If you do a good enough job, eventually you will have people come and complain about your rules.
Me, I'm happy here on SOL but I have a well-received story on ASSTR that can't come here because it revolves around a woman, now of age, who became a sex-slave when much younger. It doesn't matter how the subject is treated. It isn't allowed on SOL.
Follow their rules or go elsewhere. It's not a hard decision. Tell the webmaster, if you want. Maybe you can convince him he's wrong. Whining to the readers that you don't like the rules does not increase my respect for you.
-ZM