Sorry for taking so long to answer your question.

See Rashi in Chagigah (2b) that writes: "the Cheresh that the Chachamim spoke about in all places - that they equated him to someone who is mentally retarded and exempt from Mitzvos - is only one that neither hears nor speaks. For they had a tradition that those that neither hear nor speak are not Bnei Dei'ah."

I would translate "Bnei Dei'ah" as "those possessing awareness" (as opposed to "intelligence", because children are also not Bnei Dei'ah even though they may be exceptionally intelligent).

If this does not sufficiently answer your question, please respond back.

Shanah Tovah,

Yonasan Sigler

This is not a Psak Halachah