Bully-proof my daughter, how old do you think is ideal?

She's 5 now, and she's as gentle and shy as a normal child can be, she can't hurt a fly. I've seen her being pushed in school once by another kid, and she just stared confused by the act, as if thinking how to react to it, or what she did wrong to deserve it. She's not a cry baby, and she's got high tolerance to pain (got her finger stucked in a cabinet drawer and didn't even cry). Some parents would say less than 10 years old is not ideal. What do you martial arts parents think? Btw, I was watching a tv program where an MMA coach was teaching kids as young as 4 years old that it's okay to punch the other kid if they get punch. Really!?