When should a teenager start buying his own toiletries?
When a teenager has cosmetics, soaps, shampoos, shaving cream, etc. that he wants of a specific brand and desires to use exclusively, then it is time to start purchasing them on his own. For example, if he uses the same shampoo that most of the rest of the family uses, then there is no need for purchasing his own. However, if he has his own variety that he doesn't want his younger brother touching, then he needs to purchase his own.
If he is younger than sixteen give him an allowance from which he can purchase personal items, such as clothes and toiletries. Do not make the allowance so large that he can buy anything he wants, but try to keep it at the level that you would have normally spent on him anyway. If it is insufficient to buy his particular brand, oh well. Encourage him to find some odd jobs or perhaps select a brand that is not so expensive.
Once he reaches the age where he can work on his own, his personal expenses should be shifted from your pocketbook to his. You may wish to supplement or make gift purchases, but he needs to learn to shoulder the day-to-day burdens.