naming of parts

At what age do you suggest to a boy a name for his testes? I was doing the bedtime routine with J last night and I was putting his clothes in the washbasket while he was having a wee, when he said "I've got balls in there". I thought he'd dropped something in the toilet (though I didn't remember him bringing any toys into the bathroom), but he hadn't, so I asked what he'd said, just to check. "I'...