There is a very nice expression in french "Ítre bien dans sa peau" which literally means 'to be well in one's skin'. It has no equivalent (that I know of) in English. It translates a sense of well being, and inner peace. Usually when we refer to adolescents we say that they are not
"bien dans leur peau" because they are in a constant state of restless agitation.
On a another level, being "bien dans sa peau" implies that at the present moment your mind is in your body and not wandering somewhere else. You are here, and you feel well.