It's not like Blockland is explicitely aimed at a certain age-group.
It's not really aimed at an age group, as it is more at a fan-group.
If you're interested in Lego or building-blocks or sandbox games, then that's who Blockland is aimed at.
The "mature" thing just comes from the users who play Blockland.
Once you get down to actually playing online, most people make a server, and then aim that towards their own different "markets".
For example, if I'm hosting a free-build, then I don't care what age group I get. If I'm hosting a serious roleplay, then I only want mature people. If I'm hosting a serious deathmatch or teamdeathmatch, then I might not want only casual players, but rather serious players.
Blockland itself is aimed at anyone who likes Lego and sandbox games.
Individual servers are aimed at more specific audiences.
And the forums, well, certain areas are aimed at certain people.
Off Topic can be slightly more adult than expected, but that doesn't exactly make it more mature.
If it were really a problem that members of these forums weren't always inviting to certain types of people, then it's really the Mod's problem in that they should put a stop to it.