Hmmm.... I would definitely do all the desensitizing that you can with touching and leading, feet, and small stuff, but if I'm not mistaken isn't join up usually in the round pen with circling and driving the horse forward, ect (i know it involves body language from both horse and person) and a lot of mental capability? In my opinion...might be a bit much for a weanling, but I'm all for letting them grow and mature at a nice rate. The circling will probably be a bit much for the joints. And maybe the pressure of driving them might not "click" as they are so young....
Well, since join-up is pretty mcuh them asking to join your "herd", I woulddn;t do it until they are at the age where they might be pushed out of a herd(like stallions do to the older colts). But I could be wrong
Right after I posted it, I did some more research. I was pretty sure it was more for an older horse and found out I was correct. Thanks for saying something..lets me know I was on the right track.