How do I set a maturity level?

[Loeki Werefox]

How do I set a maturity level in Marketplace?

 

[Rolig Loon]

Use the drop-down menu at the top/center of the screen (assuming that the RL age you declared to Linden Lab is actually over 18).

[Lindal Kidd]

You set your personal Maturity preferences right at the very top, with the pulldown box in the center.

You set the Maturity level of a product listing in My Marketplace/Merchant Home/My Listings, then use Actions at the right to edit your listing.  You should see this window, which includes a place to set the item's Maturity level.