Piaget described four stages of cognitive development. If you have not studied these, or need a refresher, take a look at the video.
This category can be sub-divided further into MLD (moderate learning difficulties), SLD (severe learning difficulties), and PMLD (profound and multiple learning difficulties).
In the UK, children with MLD are educated in mainstream schools, ideally supported by an Education, Health, and Care Plan (EHCP), or special schools (places are very limited). Children with MLD are usually identified in primary school, and an application for an EHCP may be made. The school SENCO will work with parents and Local Authority to suggest an application for a special school place, should they feel that a secondary mainstream school could not effectively support the child's learning, even with the provision of an EHCP.
Children with MLD will have difficulty accessing the school curriculum across most areas, and learn at a slower pace than their peers, even with appropriate differentiation.