Piaget suggested there are four stages of development, rigid in sequence (since questioned), and hierarchical, moving from sensory through to abstract processes. Piaget suggested that what can be learned and applied is restricted…that is until the child is at the right level, certain tasks cannot be learned or performed, cognitively.
Piagetians never looked beyond the approximate age of 18, although a few neopiagetians and newer cogntive variations have done a bit more work on adults. In my view the most relevant applications of Piaget to adults came much later.