I have heard that also about not letting a large breed do agility until full grown or at least a year old. Full grown for my breed is at around 2 years old for mental maturity and bone formation. But I have seen large breed dogs way under one year old doing agility on T.V.
Bailey will jump beautifully when prompted but Chloe is just a born jumper. Chloe is almost 8 months old and has been jumping over Bailey for a while now and he's tall. If he's in her way and she's got the "zoomies" (not often) she will jump him like a hurdle and not touch one hair on his back and she's QUITE a bit shorter than Bailey is. There's not a thing I can do about this. ;)