Kids are not a good example to compare to adults on this one - the bones in their hands/wrists and their brains are not fully formed.
Jackson wrote numbers and kept notebooks:
https://www.rrauction.com/auctions/l...ok-page/?cat=0
https://www.gottahaverockandroll.com...-LOT38717.aspx
So he clearly had the ability to hold a pen. I also imagine he drew pictures or made doodles at some point in his life, even if only as a kid.
This is all getting away from the point of this thread. And I still say that this makes a lot more sense as a legit signature than it does as a fake given the provenance, the other photos, and the period published newspaper piece that has hand-signed photos in it, not facsimile signatures.