So there's no way to contribute if you're under 18? 0:

This ask took me a while to answer, because I wanted to consult our project leader directly, first. Here is what they have to say on the subject:

Yes; the google doc is open to all ages, since we can quickly and easily remove adult content if we see it. I think the forums will be a similar case. But minors will never have their skype contacts or other contact information intentionally collected or shared, so they are not encouraged to use the google submission form. Additionally, the irc is not guaranteed to be moderated even if we recommend age appropriate behavior, so we advise discretion.

We fully intend every aspect of act 8’s final product to be age-appropriate and accessible to readers of any chronological degree, but we also care about the safety and privacy of minors in the fandom. Hopefully these avenues allow a fun way to contribute.

The google doc can be found here, and is a great way to let us know what you want to see from Act 8, make suggestions for how to tie up those pesky dangling plot threads, etc. More information on the forum and the policies thereof will be coming soon.

There are many tremendously talented young Homestuck fans out there, but the safety and privacy of minors takes precedence.