I'd decided by my mid 20's. Lots of reasons. I love children and enjoy working as a nanny, and really hope to foster one day. I know I won't have biological children though because:
There are more than enough people on this planet already. About 7bil too many in fact
I couldn't possibly afford to give them the opportunities I've had, never mind the ones I'd want them to have
I have shitty, shitty genes. It would be selfish to pass them on
I have no interest in having a relationship, I rather despise men
I don't have the energy or the immune system to cope with kids full time
I love animals and find pets meet most of my needs to nurture and be loved
I'm afraid of death, and losing the ones I love - why would I create someone else to be anxious for, and know they will most likely have the same fears
There's the risk I couldn't really connect with their personalities. I'm surprised how different I am from my family of origin. It would be horrible to have a child you wouldn't chose to be friends with if they were a stranger
I'm sure there are more reasons, but those are the ones at the forefront of my mind.