When I was 13. I don't think you really need to when you're an adult, except for the odd exceptional circumstance (like when someone attacks you). I'm not a total wus--I used to wrestle and have taken some Kung Fu, and on a good day I can bench about 250 or so--but I just don't see it as necessary. I have a couple of friends out here who get into fights routinely, but its often their fault and they could almost always have avoided them.