Guerr hit it right on the button.
You're going to be extremely hard pressed to find a LS that is strictly mature. There's always going to be one or more people who don't always want to be as such. This does not mean they aren't as whole, but if you have to "act your age" the entire time, then you're not going to have fun in this game. Especially with the things that are required, and how stressful this game can be. You're going to need to have some sort of reliever/outlet.
Also, this does not mean, that if you find something or someone offensive, that you cannot speak with a leader and either warnings or actions will be done to ensure your game experience is not hindered.
That being said, you're more than welcome to join my LS. As Guerr mentioned it's a cross between two LS's and will house both. I am not sure if PerfectFlow is still openly accepting members, but my LS is at the moment, to ensure no one gets left behind.
Please feel free to send me a /tell, and either I, myself, will deliver you a pearl, or I will find someone who can.
Hope you enjoy your stay.