Most platforms allow individuals who are 13 to use their service as long as their parent/guardian allows them to. If you parent hasn't let you, then technically it's illegal, but it's not really enforced by the platform.
I don't think he's saying that you can't post it, I think he means you should mention and credit the OP of that post, even if it was from the old forum.
This is because at 13 (on average), kids begin to mature and therefore can be trusted to make good, sensible decisions, and to not act immaturely or to mess around. Your reaction clearly shows us why people put an age limit