No age, silly; they’re ghosts!
(Kodomo wa kaze no ko;
“Children are children of the wind”)
Children sometimes seem impervious to the cold, and will happily run around and play outdoors in weather and temperatures that tend to drive adults indoors for shelter. By extension, this becomes an assertion that children should play outside even when it’s cold and/or windy instead of being kept cooped up inside.
This simple phrase begins with the noun 子供 (kodomo), “child[ren],” marked as the topic of discussion by the particle は (wa). The comment on this topic has been whittled down to a single noun phrase, comprising the nouns 風 (kaze), “wind,” and 子 (ko), “child,” joined by the associative particle の (no). One may imagine a sentence-final structure here, although it has been elided.
This phrase comes to us from an Edo-era collection of comic tales by priest and tea-ceremony master 安楽庵策伝 (Anrakuan Sakuden), the 『醒睡笑』 (Seisuishou), considered the progenitor of the rakugo storytelling art.
The saying is sometimes followed by parallel phrase 大人は火の子 (otona wa hi no ko), “adults are children of fire,” i.e. when it gets cold outside, adults will gather around sources of heat such as fireplaces.
(“Donna ni kodomo wa kaze no ko to itte mo, kooto mo nashi de, tada no youfuku ni boushi to tebukuro dake de yuki no naka de asobaseru no wa sasuga ni chotto yarisugi ja nai desu ka?”)
[“No matter how much they say that children don’t feel the cold, isn’t it really too much to let them play in the snow in just clothes plus hat and gloves, without a coat?”]