youngest

Synonyms: young, youthful, adolescent, immature, juvenile, childish, puerile, infantile
These adjectives relate to an early stage of growth or development and to its accompanying characteristics. Young is the most general, applying to various periods of life, generally before middle age, as well as to inanimate entities: a young child; a young couple; a young galaxy.
It can suggest a youthful attitude or outlook regardless of chronological age: young at heart.
Youthful suggests the positive characteristics, such as enthusiasm, freshness, or energy, that are traditionally associated with youth: approached the task with youthful ardor.
Adolescent connotes the physical and especially mental or emotional characteristics of those between childhood and maturity; it is generally not disparaging except when used of an adult: adolescent insecurity; an adolescent outburst from the trial lawyer.
Immature is more clearly judgmental, implying that someone falls short of an expected level of mental or emotional development for his or her age: an emotionally immature adult.
Juvenile suggests the immaturity usually associated with adolescents, but it can convey an attitude of tolerance as well as criticism: the juvenile pranks of the conventioneers.
Childish is similar to juvenile but with a younger frame of reference, often suggesting selfishness, stubbornness, or lack of restraint: a committee member with a childish need to have the last word.
However, it can also suggest such positive qualities of children as innocence and wholeheartedness: took childish delight in tending his garden.
Puerile and infantile are used derogatorily to suggest extreme immaturity, especially with regard to social manners: a puerile joke; an infantile boast.