How old are people when they first start being able to reason?

I vaguely recall reading somewhere in the psychological literature on the development of reasoning skills that kids are able to reason as early as 4.  However even that may underestimate how early these skills emerge.

Our daughter is 3 years and one month old.  Here’s a conversation we had this evening.

Lillian (singing): “I’m the king of the castle, and you the dirty bascal.”

Us: “Its rascal, not bascal.”

“No, its BASCAL.”

“OK, well, grown-ups say rascal.  Little babies say bascal.”

Lillian ponders for a moment, then smiles. “I’m a baby!”

She had performed the following inference:

Babies say bascal.
I say bascal.
Therefore, I’m a baby.

This of course is an invalid inference; she might be something other than a baby which also says bascal.

However logical perfection is a lot to ask of a three year old.  In context, despite not being deductively valid, it wasn’t a bad inferential move to make.

She’s been making these sorts of inferences for a while – indeed, since well before she was three.  And explicit or verbalised inference was preceded by patterns of behavior which seemed to indicate an implicit understanding of certain inferential patterns.

So we can say the following with certainty: Reasoning abilities of a rudimentary kind are, at least sometimes, exhibited by humans before they turn 3.