A piece of validation that people might need:
It is totally okay to be completely adamant that you don’t want children and then change your mind. It is fine to be adamant for years and then change your mind.
It’s not suddenly justified that people dismissed you and said “Oh you’ll change your mind when you’re older.”
This is a social norm and it’s understandable that a lot of people can’t get it through their minds that not everyone wants kids, but that doesn’t make it okay. It doesn’t make it okay to pressure you and disrespect your feelings, even if they turn out to have guessed right later.
You’re also not betraying anyone who never decides to have kids. It’s not your fault people are pushy and obnoxious about other people’s choices about children, it’s their fault they choose to act that way.
Changing your mind doesn’t mean that pestering people is reasonable or that all along your purpose in life was to reproduce, either.
They didn’t know you better than you know yourself, they just make the same guess at everyone on the planet, so they are bound to be right eventually–of course some people will change their minds.
Additionally, it’s okay to not be adamant about your feelings about children either way; people aren’t entitled to tell you how your life will turn out just because you are unsure.
It’s easy to feel like you have to be really, really certain you don’t want kids before people will even consider respecting that, but that is not fair, and you are entitled to be unsure.
Whatever you choose to do, however strongly you feel about it, however many times you change your mind, it’s always always only your business (and the business of whoever else you parent with, if you parent), whether or not you have kids.