Hi! First of all, you have all of my congratulations for taking (or beginning to take) such a decision so early. As many others have said, transition whilst young is the best,
physically. Actually, except for the male sexual organs, if you could start at about 12 years old, you would look pretty much exactly as you would have if you had
always had ovaries. On the other hand, if you started when you were 20-25, you could still look pretty close to that, but your body would retain some pretty obviously
male characteristics that cannot disappear at all (wide shoulders, higher height, narrow hips, voice...) and others that would require a big effort to fix (Adam's apple,
male face, facial hair, etc.) Finally, starting after that will leave you with even more, harder to undo, unfortunate changes. Also, the younger you start,
the easier it is, socially.
Also note that if HRT is initiated DURING puberty, there have been many stories (though no real study, so doctors might not believe you) of some male characteristics
regressing and some female ones to appear. For example, a member here who started at 17 reported her hip bones getting a bit wider and her nose bridge shrinking.
So basically, every day you are off HRT is a day when your body strays farther and farther from what you want it to. Believe me, I am only 20, and I cannot express in
words how much I wish I had started sooner. Also, do not believe it's "before 18", then nothing matters. Puberty "continues" until you are 21~25, and on top of that,
testosterone causes ongoing "damage" (well, it's not really damage objectively speaking, but...) to your body all through your life, making your bones heavier and
more male-looking every year and slowly making you bald.
Plus, if you really are a girl inside, your inappropriate body is likely to destroy your mood gradually over the years. This may or may not bring you to a suicidal
depression, drugs, etc. but is almost certain to bring you great sadness.
Purely physically-wise, there is no reason to delay HRT.
However, many reasons may (will, actually) bring people around you (parents, therapists, doctors) to try to put the process on hold until you are an adult:
-They will want to make sure you do not make a bad decision (whilst most transsexuals have no change of mind regarding this after their puberty begins, there is still
a possibility that it was a phase or something else).
-They will think it is not urgent (it is very hard to make people understand how pressing this matter is).
-They will want you to be psychologically ready (as transition is hard to get through - you need to be accepted and many other things - many doctors will require you to
be 110% functional and problem-free, mentalities. Of course, doctors vary. This, whilst justified (there have been cases of people having a depression or
de-transitioning from not being able to deal with the ordeal), is extremely frustrating. Yeah, I have been trying to transition for about two years...)
Now, for money. Depending on where you live, costs vary:
-Hellish Lands (ex.: United States): Everything must be paid for, unless you are one of the few privileged to have good very good insurance, in which case HRT only is
covered. Transition can then cost more than 100 000 $.
-Not-too-bad Lands (ex.: Québec, Canada): Sex reassignment surgery is free. Doctors charge no fees for consultations. Hormones cost 80% of their basic price
(can be free if you are extremely poor). The rest, however, must be paid for (therapy, hair removal, FFS, Adam's apple reduction, speech therapy, etc.)
Transition can then cost 10 000 - 50 000 $, depending on your needs.
-Dream Lands (ex.: Finland): Everything deemed "necessary to achieve a female appearance", sometimes even including breast augmentation, is free. Therapy is free,
and hair removal too.
Better places in terms of costs, however, have a drawback: in places where transition is partly or entirely paid by the state, you will have to fight a lot and for a long
time to have your prescriptions, etc. because the government wants to avoid unnecessary costs, whilst in places like the US, it is basically the chaos and since you're
the one who pays, the only thing that can keep you back is a doctor's opinion, and you can simply change doctors if you want to.
In any case, the first step into transition is definitely telling your parents and/or a therapist. I hope your parents are understanding.