In reply to Only a hill:
It's a western dilemma! A few years ago, I watched a Nepali girl of about 5 or 6 peel potatoes with a kukuri, while her baby brother was strapped to her back.
My 7 yr old uses a safety knife, (I teach bushcraft and safe knife work) Hultafors make a reasonable safety knife, with a blunt point, stainless blade and hard plastic sheath. All for the bargain price of £5. A penknife is a liability, easily causing quite a nasty injury as they fold on the users fingers, Opinel knives aren't much better in this respect.
It may seem counter intuitive to some, but the sharper a knife is the safer it is, as you need to apply less force, so any cutting action is more controlled. This said I would never let my 7 yr old use his knife unsupervised, that would be asking for trouble as he is just not mature enough not to injure himself through a lack of concentration.
A knife is a tool, ok the American gun butters use the same argument, but a knife really is an essential tool used in its various forms by every human being on the planet, for at least the last hundred thousand years. To suggest otherwise is just idiotic.
So yes, kids should be able to have penknives (although a sheath knife is safer), after a little tuition on their safe use.
Adam