Casting for age authenticity

When casting the voice of a child in any radio commercial or production, it's usually best to cast an actual kid.  While listening to some old radio dramas recently, I was dismayed by adults' attempts at pretending to be kids.  Their unconvincing efforts took me right out of the story each time.  An accomplished voice actor might be able to pull off a child's voice convincingly, but those instances are exceptions.  When a commercial or production is well-cast, the audience shouldn't think, "That's a 50-year-old attempting to speak the way (s)he thinks eight-year-olds speak."