I think that is old enough, Swiss Army knives are very good to learn on but I would cation against the the super tiny ones like the classic they have no lock, and the handel is too small too hold on too even for a child's hands.
I would sugest hitting up a Big-5 (or any sporting good store) and get simple inexpensive folder ($10 -$15)(like M-Tech, V-tech, Smith and Wesson, or Bucks and Gerbers) but they have a lock (often a liner lock) and the handel is big enough to hold on too.
My dad let my little brother use his Ratwiler at age 8.
My $0.02 is to teach that a knife is a tool and any tool is dangerous if used irresponsiblity.