Learning a style of dance is a lot like learning a language. It comes much more easily to some people than others, and there are very different expectations for dancers of different ages. A high school English class is structured nothing like a kindergarten reading lesson. Thus, a pre-ballet class is very dissimilar to advanced pre-professional ballet.
Young children must learn the alphabet before they learn to read, and their young minds learn best through repetition and fun activities. They are not expected to memorize complex sequences or execute movements with precision. Dance steps are often taught in the context of games and imagination exercises. The main goal is to set a base of fundamental knowledge of dance movements that can be expanded with further education.
Older elementary aged children who have the basics of reading begin to work on comprehension. School lessons heavily feature activities for identifying and retelling parts of stories. In dance, these students usually know their basic steps and are now learning to put them together and memorize them in new combinations. Just as school children read longer and more complicated stories as they age, dance students learn more difficult, faster, and longer chains of steps as well.
When students reach high school, they are able to fluently read long and challenging passages like Shakespeare's plays. The focus of English class is no longer getting through the text, but analyzing the meaning behind it. Dance students in their teens with many years of training behind them usually spend their class time developing artistry. They have already learned their steps, so they work on tiny aspects of technique that allow them to emulate a choreographer's vision or a character's emotion.
For adults beginning a new style of dance, the process is more like a person who is already fluent in reading and speaking English choosing to learn a new language, like Japanese. The student already understands the way alphabet builds language, but they still need to learn the letters and sounds before they can begin to read. Adult minds can handle structured lessons that require memorization, so even though beginning adult ballet students learn the same steps as little children, their classes are set up differently (often more like older elementary age level classes). Just as there is no expectation for a native English speaker to know any Japanese at the beginning of a language course, there isn't an expectation for a new adult dancer to know any of the basics. However, adult dancers are able to remember and execute steps with more precision than young children, and their classes often move quickly.
Regarding prodigies and extremely talented child dancers (such as young Misty Copeland and Maddie Ziegler), they are very similar to children gifted in academic areas. Some kids read fluently at age 4, and some 5th graders easily understand college-level physics. It's pretty uncommon, but sometimes children excel far beyond what's expected for their age. All child prodigies should work at the upper limit of their abilities some of the time. It's acceptable and good for an amazingly talented 10-year-old to dance in the same class with high school seniors if she can perform at that level. It's also a good thing for such children to work with others their age sometimes. Even a prodigy needs to wait until her feet are strong and developed enough before starting pointe. Always working at such a high level can be exhausting for children, and sometimes there's a disparity between what the body and mind are capable of. Balancing time among ability-appropriate and age-appropriate classes helps build a well-rounded dancer.