Something modest, well made and maintainable rather than large or dramatic. A three-inch folder with a drop point or sheepsfoot blade in an easy-to-sharpen steel, plus a stone and ten minutes of teaching. Check your state's rules on transferring a knife to a young person first.
A first knife that gets carried is worth more than one that impresses and sits in a drawer. Three inches, a pocket clip, a drop point or a sheepsfoot, and a steel that sharpens easily on a basic stone.
8Cr13MoV or 14C28N are ideal here: forgiving to learn on and quick to bring back.
Give a sharpener with it and show them how to use it. Someone who maintains a tool respects it, and the maintenance is where the relationship with a knife actually forms.
Cover the four rules while you are at it. See teaching safe knife handling.
There is no federal minimum purchase age for ordinary knives, but several states regulate sale or transfer to minors and the ages vary. If the recipient is under 18, confirm what applies where you are.