I'm 53, started HRT aged 51. I've been happy with the facial changes.
Someone who says you'll never be a real woman is either mean or ignorant. Either way doesn't reflect well on them, right?
You are real. You are a woman. Therefore, you are a real woman. Period.
If you feel driven to educate them, you might explain how everyone is entitled to their gender, and that when you are living as a female, that is your gender. You don't take kindly to people who want to take your gender away from you.