My 13-year-old son wants to start running, and has asked for some weights. What are the guidelines? I'm worried about injury at a time when he's growing so rapidly.

Fitness and health guidelines for adolescents focus on three areas - general physical activity, vigorous aerobic activity and strength training - and recommend adolescents to be physically active nearly every day for an hour or more. Encourage your son to engage in a cross-section of physical activities that he enjoys, and that involve a variety of different muscle groups, because the evidence shows that physical activity levels decline during adolescence, a trend that continues into adulthood.

Teenagers should pursue vigorous exercise for at least 20 minutes at a time and three times a week: try jogging, basketball, racquet sports, football, swimming and cycling; mundane activities such as stairclimbing are great, too. Vigorous activity enhances mental health and increases the good, high-density lipoprotein cholesterol as well as heart and lung aerobic capacity.

For years, weight-training was not recommended for children and adolescents because it was thought to interfere with bone growth and cause joint injury, but we now know that in moderation it is both safe and effective as part of a comprehensive fitness programme, provided it is performed in the right way and under adult supervision - treat your son to lessons with a qualified instructor in the new year. However, he should avoid intensive weightlifting, power-lifting and bodybuilding until he is at least 15.

· Joanna Hall is a fitness expert (joannahall.com).

Send your exercise questions to Weekend, 119 Farringdon Road, London EC1R 3ER (weekend@theguardian.com).