The efficiency of tai chi chuan.how effective is tai chi chuan when facing other martial arts?

Well i have practiced tai chi chuan since i was 8 years old taught by my grandfather.I have recently interest in the martial side of tai chi chuan.I have recently spar a muay thai pratitioner who have 5 years under his belt and won.But i couldn't defeat my grandfather in sparing as well as my cousin who is a jujitsu pratitioner who have 10 years under his belt.My question is tai chi chuan only effective against "hard" style martial arts?My grandfather also told me a a tai chi chuan pratitioner grow more powerful as they grow old.Is it possible an old man/woman fight with ta chi chuan?Please share insight,experience and opinion.Thanks!!!