In Didascalia 2.1 we read that Solomon began to reign when he was twelve years of age. The source of this is not scriptural, though I had no idea whence the idea might have derived.
Subseqently, in investigating parallels between the Apostolic constitutions and ps-Ignatius, both of whom also make this statement, I found that Genesis Rabbah 11 makes the same statement. Is it possible that this is part of common haggadic discourse, and that this is the Didascalist’s source? As such this would hardly be surprising, as there are other examples of this. However, the blog allows me at least to made the addendum.