Learn Russian: В детстве он был немым.

When describing someone's state or identity in the past using был (was), Russian usually puts the following noun or adjective into the instrumental case. This is especially true for conditions that are seen as temporary or tied to a specific phase of life, like being mute during childhood.

Why doesn't the sentence say в его детстве to mean "in his childhood"?

Russian frequently drops possessive pronouns like его (his) when the context makes it obvious who is being talked about. Because the subject of the sentence is он (he), it is naturally understood that it was his own childhood. Adding его here would sound unnatural.

Is there a historical connection between the word немым (mute) and the Russian word for "German"?

Yes! The adjective немой (mute) is the root of the noun немец (a German person). Historically, Russian speakers used the word немец to describe any foreigner who could not speak Russian, as if they were "mute." Over time, the meaning narrowed to refer specifically to Germans.

Why do we use the prepositional case here (в детстве)?

The phrase uses the preposition в (in) followed by the prepositional case ending -е to indicate a period of time. Just like in English, you use "in" for stages of life in Russian, such as в детстве (in childhood) or в молодости (in youth).