The terms “dumb” and “stupid” or “feeble minded” are inexact and have become quite insulting. It also is a very complex subject since few individuals are consistently deficient, e.g., someone who has trouble with understanding to read or knowing how to act in common social situations, may turn out to be quite advanced in other areas such as music or math. Referring to “mental disabilities” is deliberately inexact since the situations are inexact.
“Development disabilities” refers to any sort of mental or physical problem that dates back to birth or before, or shortly thereafter as opposed to something that developed later in life such as disabilities relating to disease or injury.