How did you know you were childfree?

Basically the title. How did you know you didn’t want kids?
This isn’t meant to be rude or anything, I am also kinda sorta? childfree (idk). I am just curious about other peoples experiences.

I feel as if I cannot predict the future and if 30 year old me will want biological kids (realistically I won’t but people always say how people change. Adoption has always been on the table but NEVER bio kids)

I’m in my 20s and currently pursuing 3 difficult majors. I do also plan to pursue a doctorate which will keep me in school until I am at least 28 so again bio kids just aren’t plausible andddd I’m a lesbian so it’s not like the child would have my dna and my wife’s. I don’t know a single person who is childfree. The one person I knew who was decided to have a child because it “looked fun” (her exact words.) When I see her and look at how her life is post child, i definitely don’t want that 😭 and it doesn’t help that she had her child pretty late in life.
Thanks!