Oh, I have no idea what the best answer is, but I think it's prudent for us to wait until Louis is 2 before he pulls any weight, given what we know of his breed and the time it takes for maturity. If I'm incorrect, and it would have been fine for him to starting pulling at a year, I'm not really out anything but time. The consequences for erring on the early side though, are potentially more dramatic.
I'm also quite sensitive to this subject, as my husband is a surgeon and sees plenty of children with injuries from repetitive motion, and strain on immature skeletal structures. Children! Not only is it painful for them now, they have a lifetime of discomfort and disability ahead of them as a result of too strenuous training and activity early in life.
Add to this my 32 yr old SIL has just undergone back surgery to try to reduce the chronic pain she's been living with due to significant time spent as a young child in gymnastics, and then diving.
All this to say, it's definitely top of mind for me.