I am unfamiliar with 5th edition, but in Pathfinder and 3rd edition D&D, the threshold is as such:

Intelligence score of 1 or 2 (no creature with an Intelligence score of 3 or higher can be an animal).

http://www.d20pfsrd.com/bestiary/monster-listings/animals

The absolute minimum intelligence for people is 3.

Also keep in mind - intelligence has nothing to do with 'mental health'. You can have someone with intelligence 25 and 'poor mental health' (mad scientist), and someone with intelligence 3 who's stable as a rock (Gentle Giant). Intelligence has to do with book learning, for the most part.

I am unfamiliar with 5th edition, but in Pathfinder and 3rd edition D&D, the threshold is as such:

Intelligence score of 1 or 2 (no creature with an Intelligence score of 3 or higher can be an animal).

http://www.d20pfsrd.com/bestiary/monster-listings/animals

The absolute minimum intelligence for people is 3.

Also keep in mind - intelligence has nothing to do with 'mental health'. You can have someone with intelligence 25 and 'poor mental health' (mad scientist), and someone with intelligence 3 who's stable as a rock (Gentle Giant). Intelligence has to do with book learning, for the most part.