It depends on the people. I'd say you have to be financially and emotionally stable, in a loving and commited relationship that has lasted at least 2-3 years with you both successfully living together.
As for age, over 20 and the complications with birth decrease. Over 30 and the woman's fertility starts to rapidly decrease every year. So between 20 and 30 is the safest and most successful time to try for a baby.
Personally, I want to have my own house, my own car, and be either engaged or married before I start trying for a baby. I also want to be stable in my career and at a point where I could take maternity leave without a problem. For me, this will probably be around 28, I will graduate from medical school at 26 and want to complete my first two years of foundation training before I start trying. I would like to have my first baby before I am 30.
It's fun to think about this stuff