My nephews got their first PW40 at 4 I think. Neither especially tall. We just wound the throttle screw right in until falling off didn't matter, then wound it out as they got better.
Both of my cousins had PW50s at a little over 2 years old (with stabilizers to start with), they now have numerous British Youth Trials Championships between them so it's never too early!
I had a QR50 (I think, the Honda Monkeybike version) when I was four.
Genuinely think the earlier you start the easier it is. Have you looked at the electric trials bikes? They look awesome, not sure if they're for slightly older kids though?