We started working with our calf at one week old, but just with us standing in cow riding position.
At 5 months old, we "ride" with me using my feet as training wheels, and on uneven ground she has lifted me off my feet for a second or two. Standing, if she is standing with good balane, she can hold my 150 lbs easily for 5 - 6 seconds. She does this comfortably for my 162 lb husband for 3 - 5 seconds.
At this point, we are still working on getting her used to the position and the feeling of weight, but not asking her to support my weight for any length of time. As she grows a bit more, she'll need to take more weight as only my tip toes will be able to touch the ground. We expect that once my feet are off the ground completely, she will be capable of supporting me for short rides on even terrain, and then we'll go from there.
As long as we keep working at a gentle pace and have an animal that is showing no apprehension about our daily "rides" (and willing coming up to us when she sees we have the halter) we figure we are working within her abilities.
Time now for our evening halter hike, foot handling, and a bit of cow riding training.
Lynda