The way I was taught, the turn is initiated 40Kts.
Reason: The last thing you want, is to start sliding towards the ground while turning.
And of course, because it looks so neat to do it low, a lot of mishaps happen when the ground is a bit closer than you thought it might be.
Check this to see what I mean.
http://www.verticalreference.com/Vid...angerPilot.wmv