I'm a staff member of Avendar: the Crucible of Legends, and will recommend it to you. Avendar meets the criteria you posted, with the exception of an age restriction for staffers. Though we don't have such a policy, and had an unusually mature 16-year-old staffer years ago, all our current 10+ staffers are 18 or over, with the majority being post-University professionals. I'd guess the average age is 26-27.

A topic you will probably want to know about when approaching a new MUD with the goal of becoming a builder is what kind of building tools the MUD has. The building tools in Avendar have had extensive development, with a highly modified OLC and a vastly expanded mob/obj/room prog/script system. The progging/scripting system allows much creativity and interactivity (for example, I'm currently building an area which has a prog-generated sub-area for "battle training", with a randomized layout templates and randomized desc/mob/obj themes).

We require our staff members (including builders) to show some dedication to the game and demonstrate their RP ability; they must also be familiar with the spirit of the game and the players' perspective; thus, you'd need to play a character to level 51 (hero) before submitting a formal application to the staff.

As Dovolente mentioned, w/ thier MUD, I know we're always looking for good builders on AU, but we recruit exclusivly from our player-base.

Ditto on playing as a prereq here.

Actually, I'd be wary of a game that fit your criteria (well-established, large mature staff, etc.) that accepted builders that haven't played their game much. As an experienced builder, you probably appreciate the difference between making a standalone area vs. making part of a world, and it's hard to do the latter without some legwork as a player.

Not to be a 'me too', but yeah, all games I can think of offhand that fit your criteria require you to play their game for X amount of time first.

I think they are all pretty accurate in their assessments. The best thing would be to either start playing a game you might want to build for, or drop the 2 year requirement.

For example, first game , meets all of your requirements, except for the 2 years one. We're still in development, so will accept anyone that has good writing skills, is 18 or older, and seems to be someone that can dedicate some time to the project.