When I first began studying child development I thought things happened in a pretty linear fashion. First you learn to crawl, then you walk, after that you learn to talk and so on. And if you study information about ages and stages, you can come to the same misinformed conclusion.
But after 20 years of experience with young people, I can tell you for certain, learning is not linear. Children might learn and grow in one area and retreat and regress in another and they can do it all at the very same time. When we forget about the non-linear life experience of learning and growing, we set ourselves up for disappointment and we have unrealistic expectations of our children.