These are some of the rules that I generally enforce in my games. Some may consider them strict, but I am lenient. Mostly they are guidelines.
If you have any similar rules, please feel free to add them.
1. THOU SHALT NOT ENGAGE YOUR FELLOW PCs IN MORTAL COMBAT, unless agreed upon by both players and if it will advance the story in some way
Successful doesn't mean good.
You mean less than 14 years
I got into a fight with another player once, but he was asking for it, he's been nothing but rude to the whole table since the moment he walked into my friend's house, hogged all the drinks and snacks and hit on one of our female players very inappropriately to the point that she went to the toilet to calm down. So my friend asks him to leave, and this guy takes a swing at him. My friend has brittle bones disease so a mild tap can actually break his bones, so I proceed to use the guy as an impromptu broom and wipe the floor with him. Never regretted it for a second.