As for parental support, assess what the risk is in coming out to your parents. Worst case scenario is they throw you out and disown you. If that's the case, and yes it sucks, you need to suck it up and finish out high school though make a plan for transitioning in/after college. There are enough uneducated young MtFs out there that you don't need to be one more.
If you think its safe to open up to your parents, take it in steps. As good as it might feel to tell the truth, whole truth and nothing but the truth, you might as well threaten to hold your breath until you get your way. They might be the 1 in 1 million parents that immediately take you for a completely new wardrobe and start calling you "Amy" on the spot, but they're probably not. As parents you need to give them time to warm up to the idea.
So what can you do? You said you're close with your mom, open a dialogue. Talk about the abstract issues that go along with being transgendered, the anxiety, the depression, the low self esteem, etc. These are all perfectly acceptable reasons to see a therapist and they're not nearly as shocking as "I'm a girl!". Advocate for yourself, I wish I'd done a better job of it when I was your age.
If you're lucky enough to get in with an understanding therapist and sympathetic parents and doctors, go for it! But don't worry if you don't. Everyone transitions when they're ready. Don't let it become a race, take your time and do things the right way. As for the under 25 thing. The only link I've been able to find is human growth hormone (which you naturally stop secreting at 25) and if that's the case its a carrier for T not E, so it would seem to be exactly the opposite for MtFs (and good news for FtMs). Again transition when you're ready and you'll pass. I can show you 16 y/o's that don't pass and 60 y/o's that do.