I agree with Chris. DS8 is HG+, and one of the earliest signs was that logic worked with him. If he understood what exactly the bounds of the rule were and why there was a rule, he'd follow it happily. He was in the neighborhood of 8mos. old at the time. It was very handy!
Personalities affect responses, too, though. If you have a kid who isn't into logic, reasoning may not work and the child might still be PG. Don't read too much into it, you know?
But I completely agree that reasoning is always worth a try. What can it hurt?
