It depends on the child, and your exact situation; but a safe bet would be a young teenager, 13-16. An extremely responsible child in a neighborhood with very little crime, I might say as young as 10. Then again, I know some kids who aren't responsible enough to be alone before they turn 14; and some neighborhoods I've lived in where I wouldn't let the best kid stay even then. My best advice is if you trust them to fix a healthy meal for themselves (especially since they'll be alone until between 6 & 9 PM) then you can probably trust them to be alone.