Why can't I see the 18+ or 17- forums?
Most likely, the staff doesn't know whether you're an adult or a minor.  Just tell a staff member and you'll be labeled properly and will be granted access to those forums.

Do I need to prove my age?

No, we'll take your word for it.  Most websites work that way.  Note that this means the following:

• If you are under 18 and claim to be over 18, you may be exposed to sexually explicit images in the 18+ forum.  We are not responsible for such incidents if you lied about your age to get into that forum.
• We can't be 100% sure that everyone in the 17- forum truly is a minor.  Please exercise some caution.

I turned 18 and I forgot that this meant that I should be switched to an Adult.  Am I in trouble?

No, we understand that your first thought on your 18th birthday wasn't "I'd better tell the ACF staff."  However, if you continue to participate in the 17- forum well beyond your 18th birthday, the moderating team may take action.