It's tricky.
I swore blind that I would remain childfree. It wasn't for me, and I hated the patronising crap about how I might change my mind. It made me so angry, it was my risk to take, and I knew I didn't want children from the age of about 5. Also I literally have a phobia of pregnancy and childbirth, and invested a lot of time into a career that isn't very family friendly.
I'm now in my late 30s with 2 children, and contemplating a third (instill have my phobia, but it can be managed), and having children has been the best decision I made.
So I was one of those that changed their mind. And not from ambivalence to wanting children, but from feeling very strongly about it to wanting children.
I do still feel though, that as an adult, it's your decision to make, as long as you accept that the concequences are yours too.
It fills me with horror, at the thought that I could have been sterilised, and that this path I have now chosen would be shut to me, but the ability to make those choices is important.