In Chinese, there is a four-character phrase, 竹马故友 which could be translated word for word into:
Bamboo Horse Old Friend: Friends who played together riding toy horses made of bamboo.
I know there is a similar term in Japanese and Korean. Japanese is 竹馬の友 which means a friend of bamboo horse. I heard from a Japanese expert that it is not very common, but not difficult to understand for Japanese.
I wonder how a native speaker of English describes a best friend since childhood. For example, when you introduce him or her, you could say:
(S)he is my best friend since childhood. We are [best friend since childhood] or We used to [do something] together.
Is there any suitable term or idiom?