I teach kids.... but not quite that young. I also show my nephew how to play, and he's 6 - but it's not proper lessons, just basic chords and fun - he used to play the ukelele. Very very simple melodies.

I started playing when I was 5 on a uke, but didn't take lessons on guitar until I was 8.

I would say, take the lesson, but make sure to make the parent(s) aware that progress may be slow, if it happens at all... unless you don't want to, then just use the 'too busy' excuse. make sure the lesson is short. My nephew starts to get restless after about 15 minutes, and I don't even offer hour lessons to anyone until their teens.

I don't know what the real minimum age is - if the kid wants to learn, he will. If his parents want him to learn, that may not be enough.