Well, there's no reason a trans girl who gets the surgery during or after puberty wouldn't pass as a woman. But yes, you're absolutely right. I've read before about studies into that, the effects and benefits of a minor getting sex change surgery before puberty begins (or alternatively, receiving puberty blocking medication until the surgery can be performed to prevent puberty from beginning). Keep in mind that the treatments and surgeries are never done without full parental consent and without the advisement and supervision of a licensed therapist who specializes in gender identity issues who has done extensive analysis of the transgender child to confirm that he or she is in fact transgender.
Puberty is when the gonads and other hormone-releasing organs in the body start to generate hormones that cause the body to start maturing into an adult, and specifically becoming sexually mature. That's seen in the form of lower voices, facial hair, and growing genitals in biological males, and breast growth and vaginal changes in biological females. Once those changes happen, it's a little more difficult to correct through sex change surgery. By doing the surgery before puberty has had a chance to make those changes, the combination of healing from surgery and the hormone replacement therapy the individual receives, which gets puberty started, the body develops more naturally in the direction of that identified and reassigned sex.
The studies I read about concluded that for a transgender individual, transitioning earlier in life and undergoing surgery before puberty takes hold was more beneficial for the successful transitioning of the individual. I'm sorry I can't provide a link to the articles about the studies I read, because that was many years ago and I have no idea what the source was.