Our teens tend to jump straight onto the general boards depending on what interests them. We definitely have younger members, but they don't hang out on the separate board much. I guess there is a pro and a con to that, because while we have adult members I think my son could learn a lot from, there are others I'd rather he never read.
My 14 year old son has no interest in message boards. He bonds with others during role playing games. That is true in real life, too; the boys don't just sit and talk. I think that is really common for the age group, and why you are likely to have a hard time finding what you are looking for. I think any support site aimed towards teens would need to incorporate cool games.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).