noun (common) (futsuumeishi)adjectival nouns or quasi-adjectives (keiyodoshi)nouns which may take the genitive case particle 'no'infancy;childhood;tender age
彼のマナーが悪いのは幼少時代のしつけが足りなかったせいだと彼らは考えた。
They attributed his bad manners to lack of training in childhood.