The year I turned sixteen I could be found after school washing diapers, scrubbing bottles, and mixing formula. I wasn't a teenage mother. My parents, after sixteen years of vain attempts, had miraculously produced a second child.
Nine years later I could again be found after (graduate) school washing diapers, but with my own infant on my hip.
In comparison with my friends who were also new mothers, I was remarkably relaxed. I knew how to hold a baby. I knew about burping. I knew about the fontanel. I knew to put my fingers between skin and diaper when fastening a diaper pin. I wasn't haunted by fears that the baby would sicken and die because I had neglected to do something. And even though in the South in the 70s breastfeeding was still considered exotic, after sterilizing and warming all those bottles for my sister, I fought the establishment and breastfed.
Partly due to blind luck, but partly, I suspect, because of my confidence, my daughters were easy-going babies. They slept through the night at four weeks*. They didn't have colic. They didn't catch colds. There was the occasional unexplained crying jag, but on the whole they were responsive and I would even say responsible infants.
Looking back, I now see that in caring for my sister had gone through a training for parenthood that is common in many mammals. Adolescent wolves stay home and watch the new litter when the parents go hunting. Teenage marmosets carry their little brothers and sisters around and take them back to the mother for feeding. That is how they learn the art of parenting. 
Deprived of that opportunity, they neglect their own offspring.
In the large families of earlier times it was usual for girls to help care for infant siblings. But people have far fewer children now (and a good thing, too) and closely spaced, and the daily opportunity for teenagers to practice parenting is gone.
This is not to say that the experience was all roses for me. Although I enjoyed my sister, I resented the changes that her presence brought into my life, such as having to live at home while attending college so I could help take care of her. None of my friends had to wash diapers or sterilize bottles. I wanted to stay up nights discussing Sartre in the dorm, not home taking care of a baby.
In the end, it all paid off. I'm glad my life was enriched by something that, in their wisdom, many animals practice, and I was lucky to launch into parenthood with more than good intentions and a copy of Dr. Spock.
I'm wondering how many of you had the “mammalian” experience of taking care of much younger siblings, and how you felt about it.
*Part of that willingness to sleep through the night was due to the spoonful of cereal that, in those barbarous times, pediatricians advised giving babies starting at age one month.
