Alright everyone, sorry for the placeholder comment, as I went on to read the article and discover some... selective editing on @numberwang's part.
I say selective editing, but really it looks like straight up lying.

See, if you read the whole article you'd see it's far less focused on hormone therapy then it is on the experience of coming out as transgender, and how James and his parents dealt with it (and to a lesser extent Olivia).
This surprised me, as it's quite well known that starting hormone therapy at 9y is literally impossible, so I originally thought this was some groundbreaking new test trial or something.
Turns out @numberwang "forgot" this little part of the article, the only part concerned with hormone therapy 

"Puberty is one of the scariest things in the world to James. He talks about it a lot, and worries sometimes whether his armpits smell because he thinks that might be a sign it’s all starting. Puberty, he knows, is when his body won’t really be his anymore.

For now, the Oakland office of UCSF’s Child and Adolescent Gender Clinic is monitoring James for the first signs of puberty. They do this through blood work and by keeping an eye out for breast development. Once the doctor determines that puberty has begun, they’ll give him and his parents the option of his going on a hormone blocker — essentially freezing his development in place. Its effects are fully reversible.

If he goes on blockers, James can stay on them until he reaches an age when puberty can’t be avoided. If he’s still certain about his gender then, he’ll start cross-hormone therapy. Daily doses of testosterone will give him a puberty in line with his gender. His voice will deepen, he’ll grow body hair, his face will harden. Any surgical interventions, should he decide to pursue them, wouldn’t happen until much later."

You'll notice that the "California couple" has not decided anything for anyone, because no decision, not even oft the reversible kind has, or can be taken yet.  
i.e. the thread title, @numberwang main contribution is flat out wrong.
Doubly so, since it seems to implicate that therapy has been started for Olivia, which, being 5y, is just ridiculous.

And now to check the reply and reveal who has read the article to notice the obvious lie, how fun!

TL;DR: The title is false, reading the article should have cleared that up