Hmm that's weird. With Hawaii being populated with a big transgender community, me and my friends who are also trans started our transition at 14 and started HRT at the age of 16. We didn't need to go to a therapist and get a letter but maybe the rules changed since at the time we started hrt, it was the year 1999.
But even now a lot of the younger generation trans in hawaii are prescribed HRT at 16 as long as they have a guardian to sign the papers that acknowledge the prescribing doctor that the minor is allowed to take hormones. We didn't need to see a therapist or psychologist.
Maybe this is only allowed in hawaii since most transgenders know each other in hawaii but not sure how other states or countries work with their process for hrt.