For a 14-year-old boy, the recommended dumbbell weight depends largely on his experience, strength level, and physical development. The general guideline is to start with a weight that allows him to lift with proper form for about 8-12 repetitions, where the last few reps feel challenging but not painful. Typically, this could range from light weights around 1-2 kg for beginners up to about 10-20 kg for stronger or more experienced teens, but it's crucial not to lift very heavy weights before mastering technique and ensuring safety. A common practice is to start with lighter weights and gradually increase by small increments as strength improves. Experts emphasize focusing on technique first to avoid injury and recommend weights that represent around 50% of the adolescent's body weight for exercises performed with proper form and control. Teens should avoid maximal or very heavy lifting until they have fully gone through puberty and skeletal maturity is reached. For a 14-year-old boy who typically weighs around 50 kg or more, starting at a weight that is comfortable to lift for 10-15 reps without pain—often less than 50% of their body weight—is advised. It's also recommended to have supervision or coaching to ensure correct form and safe progression. In summary, a 14-year-old boy should use dumbbells light enough to perform 8-12 controlled repetitions with good form, gradually increasing weight as strength builds, typically starting around 1-2 kg up to about 10-20 kg depending on strength and experience, with a strong focus on safety and technique over lifting heavy loads.