Yes a 12 year old can do landscaping if he works with his family or family business but has to be careful. It must not be real hot outside. You need to work short times and then you can work longer as you get used to it but no more than 4-5 hours a day. You have to take regular breaks where you drink lots of water or Gatorade . You should not use electric landscaping tools yet or real sharp items into you are 14. Wear leather gloves to protect your hands when planting trees. Wear good sunscreen so you don't get sunburned and out it on often. Wear protective sunglasses and a hat with a brim to protect your head and eyes. Have a small towel with you to wipe away the sweat from your skin. Wear good shoes. Rest A LOT and again drink lots of water. Expect to get paid a good price for your hard work. About $10 per hour if you work hard. It is hard work. So you must go to bed early at night after you have showered and eaten a good healthy meal.
Must be careful not to hurt muscles by heavy lifting. Lawn mowing is hard and the impulsiveness of a young child can be dangerous. Raking, watering, planting small items are definitely things within the strength of his young body.