1. when you have a solid ( even simple ) home and income from honest work and are free of any debts and 2. when you have a partner you feel comfortable and safe - safe enough to crisis and dark times as well, a partner that shares ALL your own essential values in life and has the same goals in life that make you feel comfortable deep inside your heart and soul. and 3. when the idea of making kids and having kids is but a pleasure for your heart - all the way
To be honest with you you never really know when your ready to have kids but I do believe there is a feeling that you get when you know you are ready. Yes they are a lot of work and yes they cost a lot of money but if you feel that feeling inside you to want a child then you are ready regardless of the cost.
Try working backwards. What age do you want to retire by? At that age you should be free of the worries about paying for college. So probably a collge account for the kid should be born even before the kid is planned.