I have the answer. 14.
Both rr and Clarion had it right though.
My answer would be "When the growth rate of their feet slows enough to make the shoes affordable"
The bigger issue for me with getting kids onto clipless (Our Kid doesn't like them anyway; TGL wants them for all riding except track) was the cost of shoes. When kids' feet are still growing fast, I'm not able to afford shoes they'll be through in no time at all.
Mini-hatler now has feet just a half size down from me, so we tried him on clipless a couple of weeks ago with my oldest pair of SPD shoes.
He cycled perfectly happily around the campsite many times and successfully unclipped on both sides many times.
Went out for our first ride and got all of 300 yards down the road onto the first little rise, and over he went.
Anyhow, he's sold on the idea and I'm not getting those shoes back.