shào​nián​lǎo​chéng

young but mature

Senses

少年老成 shào​nián​lǎo​chéng young but mature

to be young in age but steady, experienced, and mature in one's conduct

少年老成 shào​nián​lǎo​chéng lacking youthful vigor

to be young but lacking the typical energy and vitality of youth; old before one's time

References

antonym 少年老成/少年老成 少不更事/少不更事
antonym 乳臭未干/乳臭未乾 少年老成/少年老成