tai chi isn't really just for old people, it's just a soft form and people go through, what I suppose could be called a kata, in slow motion which is in keeping with the idea of not meeting strength with strength. Tai chi can be devestatingly fast and deadly if that's your focus.
I remember my friend and I made our own bow and arrows and we were shooting them off in a field, his grandfather, who practiced tai chi, wandered out into where we shooting and would pluck the arrows out of the air while he was doing his forms... he would just time his movements to intercept the arrows. From the outside it looked like we timed practiced the moves together (us and the grandfather) but we were shooting the arrows randomly and he was just awesome.