yeah i started when i was about 4 or 5
it stays with you for life
Highly recommend it. Even if your kid doesn't end up being Ali or Rickson Gracie they'll come from it with more self confidence, esteem, discipline, and knowledge of the consequences of one's actions.
I was put into martial arts and have been involved in it one way or another all my life. Served me well.
*two cents *