Even older players get harassed, PvP should defiantly be turned off in most public places. Some people will still try to attack me while I'm wandering the streets, and ya'll it's just annoying when you keep getting killed but just want to go somewhere :).
This would be cool as long as it could be "hidden" as a regular building to match the city. Just because, ya know, battle arenas in the city would look odd. I really like the idea though!