I have a 3 year old Percheron/Canadian Warmblood cross. He is approx 16 hh. We've started him with ground work. I understand the knees of drafts take much longer to close, possibly until they are 5 or 6 years old. My question is, when could I start putting him over x-rails or small fences?

I do mostly lighter breeds & start cavelliti work @ 3-4 once going nicely under saddle. Normally don't add anything on ground until broke to ride, except some ground poles to walk over, so they don't overreact when asked to do first time mounted.