Babies: Keep your baby nearby as you prepare meals. Give him his own bowl and spoon to imitate what you you do.
Toddlers: Give them lettuce to tear for salads. They also love to "dump" as one of my favorite little chefs called it when she was little. Let them dump pre-measured ingredients into a bowl. Stirring and mashing are also fun.
Pre-school: Fine motor skills are developing. Mixing cake batter, whisking eggs, measuring wet and dry ingredients with your help. Many also love to start learning to crack eggs. Peeling hard boiled eggs is also fun.
Ages 6-8: Cracking eggs on their own, rolling out pizza dough, peeling vegetables, grating cheese, making meatballs. With your help, let them run small appliances like the blender, toaster, etc.
Ages 9-12: With your supervision, teach safe knife skills. If you are uncomfortable yourself, there are plenty of videos available that show you how. Remember to curl your fingers under. Let them put their own school lunch together. You might even let them use the stove. I let mine stir things on top of the ceramic stove top, but don't let them put things in and out of the oven until about age 12. Use your own judgment depending on the maturity of the child.
Teens: If they are interested and have been learning skills all along, they would probably love to attempt to prepare a full meal on their own. Encourage them to experiment with simple recipes.