I'd don't think anyone really knows when they are ready to have a child. When I was younger, I did an awful lot of babysitting and various other forms of child care. I thought I was ready.
Well, being on call 24/7/365 is a vastly different experience that nothing but parenthood can prepare you for. Your life ceases to be your own; and, the emotional rollercoaster ride - sickness, injury, bullying, bad choices, drugs, pregnancy, etc - is enough to turn you grey before your time. I was completely grey at 40.
Having said that, I had 2 daughters and I wouldn't have had it any other way.