While it’s certainly possible for a person to be stupid, I find that very often it’s more accurate to describe a person as ignorant. In this context, I don’t mean that as an insult; I mean literal ignorance, as in the described person has not been exposed to relevant information, or possibly has been conditioned not to accept that information if it is provided.

It’s anecdotal, but most stupid people I meet aren’t stupid, just missing or unable to accept certain information. This especially applies to young people.