It doesn't say anything anywhere in the Bible the minimum age for stoning.
Even given the things I said about giving little children the death penalty (and those verses could apply to a 4 year old as well as they could a 40 year old), here's an instance of God directly using capital punishment for children who mock an adult - who is not even their parent.
Quote:And he went up from thence unto Bethel: and as he was going up by the way, there came forth little children out of the city, and mocked him, and said unto him, Go up, thou bald head; go up, thou bald head. And he turned back, and looked on them, and cursed them in the name of the LORD. And there came forth two she bears out of the wood, and tare forty and two children of them. -- 2 Kings 2:23-24
Here it specifically says "little children" who were teasing the prophet. On his say-so, God sent bears who killed the children - for misbehaving as children do. This seems to warrant capital punishment.