Discourse doesn’t require you to have a certain age to use it. Communities can, depending on their content and applicable law. See Lilly’s answer below!
(this remains true tho… I hope)
The 13 years old thing is a very US-thing; many old-school forums, in other countries stated in their terms of service that you were required to be 13 years old to register, even if there was no reason for this regarding different local legislations. It was probably just because the term of services were originally written in the US, then translated, and used as it is in other language forums. It led communities outside US believe that the 13 years old was a real thing regardless the country… 