Use a tog rating appropriate to the season and the warmth of your teenager's bedroom. An all-seasons duvet would be a good investment. Use the 4.5 tog during warm summer months and the 9.0 tog duvet for spring and autumn. During winter, you can make a cosy 13.5 tog duvet by pairing up the 9.0 and 4.5 tog duvets. Teenagers sleeping in warmer homes may not need as warm a duvet, in which case, use a 9.0 tog duvet with extra blankets if necessary.
Don't forget to think about duvet fill. As so many teenagers like to sleep in at weekends, they may find themselves overheating when the central heating comes on in the morning. A wool fill duvet will help regulate their body temperature and prevent overheating.