What is appropriate age group for an altar call?

I was in an evangelical setting with some young kids and one of the helpers/ leaders basically did a sort of altar call with the kids. It actually surprised the other helpers/ leaders. What is the appropriate age for such an event / rite? If that is too much of a Truth question, then what specific advice or training has been given on the age appropriateness of "altar calls" among denominations or groups that practice them?