I had my first appointment with my gender therapist on my 30th birthday (not intentionally, that's just when he had an appointment available), so I guess I technically started in my 30s, but I had been seriously thinking about it for ten years prior.
If you want to transition, being in yours 30s doesn't really seem to be a hindrance. There's certainly nothing wrong with transitioning at a younger or older age, but it's a good balance in your 30s, in my very biased opinion.