Spirit beings have the same bodily form as mortals except that the spirit body is in perfect form (see Ether 3:16). Spirits carry with them from earth their attitudes of devotion or antagonism toward things of righteousness (see Alma 34:34). They have the same appetites and desires that they had when they lived on earth. All spirits are in adult form. They were adults before their mortal existence, and they are in adult form after death, even if they die as infants or children (see Joseph F. Smith, Gospel Doctrine, p. 455)
via LDS.org – Sunday School Chapter Detail – The Postmortal Spirit World.