I agree with Sandy in the fact that nothing really prepares you for the changes parenting brings! However, it's a huge responsibility to mentor a child throughout their formative years to become a loving, responsible and productive adult, and it's a job that's sadly sometimes under-rated. I think that it's crucial for parents to set a good example for their children, so if someone is still behaving like an immature kid themselves or has some unhealthy habits they're not prepared to give up, perhaps they're not quite ready yet. Parenting can be very tiring at times too, so if we're not healthy (physically, mentally or emotionally), it may be advisable to work on our own issues and resolve those first before we add extra complexity to our lives. A loving, committed and supportive partner is a big help too, although other family members can also provide extra support when needed.
I think parenting is not something that you can prepare for. You learn parenting as a situation arises. It is not related to age, race, financial status, etc. If one indicator can be taken into account is to be in a committed relationship before taking on the responsibility of becoming a parent, not to say that single parents cannot be great parents. It just becomes more difficult in my experience. Similarly, some people can be great parents at a young age, or not a great one in a more mature age. I guess you just have to follow your instincts.
No sure about 'ready', but I knew I wanted to be a parent when EVERYWHERE I looked there were pregnant women and children! It was like I was the only woman without a child or a baby on board! I welcomed the challenge and changes that would come.