In reply to Hans:
I work as a route setter for a wall by virtue of working at said wall. Quite a few of my mates are do the same at other walls. It just comes down to having climbed lots of routes, preferably indoor and out. Then also to be able to climb reasonably hard.
Most people that work at walls that set climb high 7s. This is because you need to be fairly strong to haul bags of holds up the walls, jug up ropes etc. Also as you mostly set and then test your routes in trainers while carrying drills, bags of bolts etc clipped to your harness. Its quite hard work.
As for being a professional setter, I think it mainly comes down to reputation. We get pro setters for some of our main lead walls. These guys can set 6 long steep routes in an afternoon from 6c-8a, test them and tidy up all in a few hours.
If you really wanted to get into it, try and get a job at a wall, get a lot of routes under your belt. Probably get up to leading at least 6c-7a. Then you could in house set the easier walls.
Becoming a pro route setter is a long hard competitive and fairly saturated road. Having said that, there are lots of new walls popping up all the time.