While I welcome the ruling I think it is safer to say born male sex you can never compete in the female category. By saying that puberty blockers to halt the production of testosterone have to have started before age 12 is just going to encourage child transition. No child is capable of making such a huge decision. Any puberty blockers taken at tanner stage 2 or earlier will result in sterility, and probably loss of any sexual function.
We're hearing more and more about abuse of children in sports, the UK gymnast story has recently surfaced. There will be unscrupulous coaches willing to implement any measure to gain success, and that will include transitioning young children.
Of course sport should be for everyone who wishes to participate so there will need to be an open category that is inclusive of transwomen and transmen. An immediate solution is two classes, female only and open. More refined solutions can be worked out as time goes by.
