I don't have any moral issue with any of these. Hollywood needs child actors, they're gonna get them from somewhere. Basically, you could start whenever you want.

They should probably start doing what they like as a kid, and really try to get their career when they become a teen.

Well I posted this because some people think children won't have a childlife. Teens, when they go to schools, like public schools, they get themselves in trouble because of like, the peer presure. And Adults, meh, most are able to go to jail and their private lifes get invaded. Thats what people tell me, I wanna know what YOU say.

Whenever the hell they want.

As long as they're looked after, then I think its fine to start performing at a young age.

i believe that a kid should just be a kid and enjoy their childhood :) and if they still show an interest sure by all means go for it, but its up to the parents if they want to expose their child/children to the limelight.

I think these days, children are being pushed into the spotlight, without truly understand what can happen to them in the future.
I believe that if somebody chooses to start a career, they should do so in their late teens to early adulthood, where they have a better understanding on what happens behind the scenes.

"but its up to the parents if they want to expose their child/children to the limelight. "

As in, you know, living through your children as you don't really have much of a life for yourself? ;)

Whenever you're talented enough to do whatever it is you do, professionally.

A Hollywood without any children would severely limit the variety of television and cinema prospects.

Childhood or teenager. For art's sake, we need children in the entertainment business. We need to find a way of protecting and nurturing child stars, rather than abolishing them altogether. And I think we are getting somewhere, now that the scandals and information of parent abuse to child stars is getting out there in the general consciousness, we are seeing more child stars who are growing up healthily and seem to be making it.

Whenever they want.