How do you deal with the "kid question"?

Ever since I got engaged I started getting bombarded with the question "When are you having to have kids?" by customers at my night job. Every time I tell them that I don't want to have them, I get the annoying response "Oh you're going to change your mind." Sometimes it gets to me a little, I'm not going to lie. I'm not a bad person for not wanting to be a mother! I noticed that very often it's women in their late 20s too, and I'm in my early 20s! I'm just wondering if there's any other women out there going through the same thing as me that might have a better answer than the ones I've been making, that's all.