You're never too old to train Martial Arts, but I'd personally prefer learning at a young age. My Grandpa trained for 16 years in Japan and held a 5th degree Black Belt in Kyokushin Karate, and he began training me from when I was 5 to when I was almost 9. I'm 22 now and since then I've trained (both from instructors and independently) in Brazilian Jiu-Jitsu, Tai Chi Chaun, Muay Thai, Wing Chun, and Boxing.