The secondary male characteristics aren't fully developed until late 20s or 30s. So, if you start at 18 there's less male specific characteristics, which is to say in artsy terms that the canvas has yet to be completely filled in as a male.
It's not strictly age related, and the answer to your question isn't a simple "if this then that." But in general, starting while in your teens will typically result in more natural and less ambiguous characteristics when presenting as female.
I'm not trying to weasel my words; I'm trying to answer broadly and generally enough that the answer is useful but not specifically predictive. However, to be clear starting earlier in life is better, especially in ones teen years.
Whatever your circumstances, if you are sure you are a trans-woman, then get the letter from a therapist and start HRT. This assumes you are willing/can to accept the life impacts of such a decision in both your private and public life.
Take good care,
Rachel